(Background, skip if bored: I am Primary Storyguide for a Saga taking place in a vaguely stereotypical "abandoned remote covenant ruins" situation. The magi have yet to spend the time developing an income source. As a result, money comes from terram vis via large balls of gold. And, with new agreement to a strict covenant charter due to vis scarcity, there's some pushback about how some magi keep much more opulent laboratories than others (the Bonisagus has a lab with Upkeep +7 in terms of the rules from Covenants, whereas everyone else is 0 or +1), and, therefore, magi should pay the covenant for their laboratory upkeep costs in vis. This led me, the OOC beancounter, to do the math to determine the relative rates of vis charges based on the numbers involved. This led me to try and take into account the cost savings of our Skill 6 Glassblower, which led me to the math below.)
So, by the rules put forth in Covenants, p.63-p.66, Craftsmen can be used to reduce the costs of a covenant. Sounds like it's great to have craftsmen, right? Here's the math, as I can figure it.
Assume a nice, young glassblower, skill 6, in a Summer Covenant. He is 3 "inhabitant points" (hereforth referred to as IP).
2 servants are needed for every 10 IP (there are some exceptions not pertinent to this discussion). Servants are 2 IP each, so our 3 IP craftsman needs .6 servants, or 1.2IP of servants.
The covenant needs teamsters to haul food and goods to this craftsman and his half servant: 1 teamster per 10 IP (again, with some exceptions that do not matter for this portion of the discussion). Teamsters, like servants, are 2 IP each, and we have 4.2 IP of people, so we need .42 teamsters, adding another .84 IP.
Thus, every craftsman added to our covenant has an actual cost of 3 + 1.2 + .84 = 5.04 IP. Because the system assumes you don't have half people running around, you actually end up rounding those numbers in the real world, so let's call it 5 IP.
For these 5IP, the benefit you get from a rare craftsman like a glassblower is a reduction in the cost of that area (in this case, laboratories) equal to his skill (in this case, 6). Thus, he saves us 6 pounds per year.
Now, on to the costs of running a covenant. Each 10 IP costs:
1 pound for buildings
2 pounds consumables
5 pounds provisions
2 pounds wages
Thus, 10IP costs... 10 pounds. And thus, our 5IP costs five pounds. Our craftsman's net savings is: 1 pound. Had he been straight out of apprenticeship, with a skill of 5, he'd've been a waste of space.
Now, that's the rare craftsman. Let's turn our gent here into a blacksmith. The IP cost is the same... you feed normal craftsmen the same as the rare ones (one leg into the trousers at a time just like everyone else, and all that). However, regular craftsman don't save you as much money: they save 1 pound + half their craft skill, round down. Thus, our skill 6 Blacksmith saves us 4 pounds, while costing us 5 pounds between himself and the infrastructure needed to support him. He won't be cost neutral until his skill is 8, and he won't be actually saving money until he's skill 10. And if he's training an apprentice to replace him when he dies, that's another mouth to feed, meaning more servants needed, who also need feeding... and chances are good that apprentice won't start out cost effective when his master dies.
Am I missing a detail somewhere in there? Is there some aspect of the math I'm not getting right? Because I've just been blithely assuming in my campaign so far that having craftsmen is good, and having been doing "what-if" fiddling with Metacreator, but my analysis here suggests to me: "skilled" (per the core rules definition of skill 6) craftsmen just aren't worth it; until you've got 275XP in your craft ability (or 10 build points in your craftsman, to look at it a different way), you're better off just going without according to the RAW.
Or, put a different way: if your covenant decides to stop living like a cheap Spring Covenant and make the place into a nice summer covenant, complete with bonus to living modifiers, kick out your craftsmen, because they have no place in your new utopia.