Nods
If you need to value items in some rule based way you can see them as investement in cost savings, income and loyalty. Covenants suggests that items that improve income are treated as income modifiers that are applied permanently. Valuing improved income is not easy as it depends a lot on what sort of income covenant has, but is relatively straightforward. Cost savings category has suggestion that every magnitude of effect saves a pound a year - which is very rough and doesn't scale with usefulness of some items (e. g. CrIg item to warm unlimited number of rooms), but gives very nice rule of thumb. Loyalty does not have straightforward rules for items that give comfort to covenfolk, but they could be treated as minor positive events that happen regularily or one time boni to loyalty. For example in an another internet based campaign Verditius that I play made couple of needles that she can use to keep clothing water resistant and warm for moon duration. She applies the effect herself to covenfolks clothes and exchanges couple of words with everyone while she does that (and of course her sigil reminds covenfolk of her while they wear their clothes). While we do not keep track of loyalty I would argue that actually improves loyalty of covenfolk as much as holding regular festivities.
Now we would need to value changes in income/loyalty to vis. For my purposes improved income and cost savings are considered equal even though rules for inflation mean they are not exactly the same. One way of valuing a pawn of vis is given in pager 69 of covenants (The Price of Vis insert) where it suggest that 10 pounds is a starting point for a negotiation. HoH:M Verditius section suggest 15 pounds IIRC. These guidelines suggest that Vis is indeed cheaper than money when bought with build points - you either could buy 10 pounds of silver with a build point or 5 pawns of vis that could be sold for 50 pounds silver (or even used to create gold worth about 800 pounds of silver).
This leaves us with valuing changes in costs or income - how much is stream of income worth? Covenants seems to value vis sources so that you can either buy vis source of one vis with 5 build points or get 25 pawns of vis in stocks with those build points. So if we assume that same 25 years as baseline for value of items that produce cost savings we can compare them to buying pounds of silver in stocks. 2 magnitudes cost 4 build points and save 2 pounds a year according to guideline - so they are worth 50 pounds of silver with that 25 year assumption. 4 build would buy 40 pounds of silver in stocks so items are better than buying silver with these assumptions.
Value of loyalty is difficult to determine. How much is it worth to a magus to have a bodyguard that would give his (or hers) life for you instead of backstabbing you? I do not venture to give a guess - we could try determining a half life of a magus and how loyalty affects that and determine worth of an magus as how much vis he can distill during his life. That excercise is beyond this post.
Magi of the covenant can do items themselves or order them from a friendly verditore. House Verditius has price guideline for items - essentially you pay 3 times what making item yourself would cost. So 2nd magnitude item would cost 3 pawns of vis (assuming someone is willing to accept such order - you are adviced to order in bulk or make such order to a magus that has an apprentice). This suggest that you can buy 20 pawns of vis with 4 build points that buy you about 6 magnitudes of effects (2 pawns of vis can be assumed to be used to pay for timely delivery by your dear local redcaps) which compares rather nicely to buying 10 levels of effect by those same build points. So buying items with build points seems excessively costly when compared to buying vis stocks. Those same build points can be used to have vis income of 2 pawns a year - half of which could be used to make that 2nd magnitude item that year (feat that should be almost possible to just gauntleted magus), but leaves us with figuring out how much time of magus is worth.
Magus time has luckily lower bound for value as magus can distill vis - assuredly very interesting and fun passtime. For a starting magus I assume that he can distill at least 3 pawns of vis (because for example having Int 3, Creo 6, Vim 6, MT 3 and Aura 3 is possible for such a young magus) so cost of item for a covenant is actually 1+3 pawn of vis. Which still makes a proposition of making an effect yourself sensible.
Overall it seems that buying silver or items is expensive compared to having vis and that items are rather expensive when compared to making them yourself or even ordering them from Verditius. Of course magi will not be able to make all items by themselves and they might value their time more than what they can distill in a season.
This all was based on rules and suggestions books give us. Some might argue that warmth and health are not really valued very much by the rules - both are almost trivial to provide with items and were not that easy to provide by 13th century technology. Having a home that stays warm no matter what is actually rather nice and I'd say that benefits of making 7th magnitude item[1] to provide that to everybody in covenant is worth probably more than 7 pounds of silver annually. No need to collect that much firewood so there are fewer workers so enviroment will stay better and there are less costs, etc. Boni to recovery from wounds and ilness are also worth a lot even just counting loss of work from workers.
None of this has taken into account that magi themselves could value comfort given by items a lot, but that is up to invidual magi.
Why I did have an idea about other covenants gifting such item to our starting covenant? It is actually a trinket when it comes to a power - its mightiest effect was Ear for Truth that would help to keep order among covenfolk (decreasing costs as corruption is more difficult to hide and over all make covenant more honest). It actually is something I'd see as something that summer/autumn covenants could give to new covenants as it is relatively easy to produce once you have lab texts and covenant that uses it will know who gave it to them - a constant reminder of your stronger allies and what they've done for you already. Rhine tribunal actually has a custom for giving gifts for new covenants.
Of course were magi able to see build points or measure them they would probably rather have vis sources - I think vis sources are currently probably best value proposition overall for a covenant - maybe with some high level high quality summae competing with them on a narrow band - almost everything can be done with magic and you can study magic from vis. Why have anything else?
Essentially I see this as question what kind of simulation do we want to run - which is question that applies to all roleplaying campaigns. If we are going for optimal optimization power over span of 50 years we have different problem than if we start with relatively realistic, but cartoonish magi in almost believable covenant. If we shoot for stars I'd recommend that we start with Aura of 9 (10 if possible), free study and affinities for our chosen arts - (12+ stress die)*1.5exp is very hard to beat and vis sources that match our arts will provide plenty of study sources. ~80exp a year (3 seasons of study) is nothing to sneeze at if kept going for even 25 years that are needed for those vis sources to break even compared to stocks of vis.
It was fun trying to write this. I'll now go over what I'll use those build points for. What do you think will be more realistic for a young magus to bring to a covenant with those build points? Some books probably and preferably a lot of vis sources (but those could be scarce).
[1] Warmth of a home CrIg 35 (Base 2 (warm to the touch), Touch +1, Room +2, Moon +3, Unlimited uses +10 levels)
I did say in that thread that we are full, though.