You can see gears shifting as Petronius switches from being focused to listening to what Korvin is asking. Taking the lab text, he starts consulting it, "Is that a complete text? Why don't you simply someone here learn it? Wait, you say there is a flaw in it..." ((What does the spell do, exactly? Technique, Form, parameters and details, please.))
"How many houses did you want to create? An why go with temporary structures when something permanent would be easier to create?"
((The spell creates Roman villa's that last for Moon Duration. The flaw makes them fall apart, pretty slowly, but still it violates Praxiteles artistic and architectural sensibilities. I'm handwaving how he knows that there is a flaw in the spell, call it a special insight of his high Finesse or something, since Finesse is still slightly necessary for Creo'd items.))
We do not see it as practical to make permanent builds to house 300 odd people. If we did that then others might expect us to host this every 7 years. it is too much to bear at this point.
(( making stuff up for the dorm sizes. If there are real numbers then let me know))
Each villa will hold approximately 30-40 people. 6-8 magi and their servants. The dorms are for the overflow. We understand that there could be upwards of 300 magi here for the Tribunal. Location and the Lotharingian debate.
Korvin has enough business sense to realize that the quality of these buildings and the size of the regio, and it's seclusion lends itself very well to being a permanent spot for a Tribunal. If the Lotharingian Tribunal is created (and he's for it, despite the Troupe's decision to not do it) and Mons Electi joins, it would be a perfect spot to host future Tribunals. It's a calculated risk that Korvin might be willing to make...
Finally, if it is a permanent site, additional funds from the Tribunal should accrue to the host; as the responsibility is no longer shared "equally." Korvin can see that pretty clearly, too.
Asked as Korvin and the player...What would be the cost to make them permanent?
"That is unfortunate," gesturing towards the Tribunal area, he adds, "after all, you have plenty of space. And it would have been much more impressive, and less time-consuming."
"You see, I already know some rituals that could have been appropriate. Conjuring the Mystic Tower and some smaller variants, for example. And as you know, one Mercurian to another, this is much less expensive in vis than for other magi."
"Spending time in the lab to invent a spell like you describe," he grimaces, "means spending time away from my research. Possibly multiple seasons, depending on exactly"
((Still waiting for the details of that spell... I can't gauge how hard it would be to invent it if I don't have the parameters you came up with.))
A tilt of the head. "Have you really thought this through? I mean, a moon duration is a bit short when one speaks of hosting Tribunal. Many magi like to arrive in advance to discuss business with sodales before and after Tribunal. And even when they do not, they often send mundane companions and servants ahead of time to adapt their living quarters to their needs. I know that in the Rhine, mundanes start arriving at Durenmar for Tribunal a month before the proceedings begin, while the last guests often depart a week or two after the proceedings have been concluded."
Asked as Korvin and the player...What would be the cost to make them permanent?
I take that back. We are talking 19-23 CrTe rituals even if Korvin casts them all and they are comparable to Wizards tower that is 76-92 pawns of vis. i do not even want to figure the cost of not having a Mercurian cast them
This is the risk... There are a number of unknowns. But is it something that a Magical Mercere from the House focused on commerce push to do? Especially when the expedient thing to do, now, given the flawed spell, is to just recreate stuff from scratch and deal with the costs as they may? Korvin is the princeps, and is faced with the pending disaster of an unready Tribunal spot. Viscaria's hubris and plans have backed Mons Electi into a bit of a corner, but there is a chance that things can be done to make it pay off.
And Petronius is Mecurian...
If you want more certainty, doing a die roll on some relevant skill, or perhaps contacting other Tribunals with permanent sites...
I've done a rough figuring of the cost and it is acceptable if Petronius knows enough of the spells that he would have have to develop many/any. what is the use of being a rich Covenant if you can't use the wealth.
So we worst case, we are talking 20 villas ( all the same design) and 3 resident halls that hold 30 magi each. I'm hand waving that the other stuff gets done.
The spell is here: https://forum.atlas-games.com/t/ooc-building-the-tribunal-field/6744/51
Finally, getting it. Of course, I'm also dangling a lot of temptations to use vis in front of the covenant. This is the challenge of the summer covenant, really. Mons Electi has income, but very little in stocks, when it comes down to it, though. Call it growing pains. 
But...you gotta spend vis to make vis.
Does it drop to a CrTe30 as a ritual?
So redesign this spell and add another for the Resident halls which are far less elaborate but not shabby.
So, what is your answer to Petronius, in character? This can be after some time consulting the other players, or you may want to ask him what his capabilities are before you make a decision. But you have to ask if you want to know...
blah blah blah about spells and stuff...
So what is the cost of redesigning this spell for the villas as a ritual and adding a second ritual for the resident halls. What is the cost of your time for designing the spells? Then we can discuss casting costs or we can share the casting between us.
It actually drops to CrTe25, Moon is +3 magnitudes.
You can change it to T:Group, or T:Group +1 and make 10/100 villas (or of Group +1, the exact number necessary so long as it is less than 100). This puts it back up to 40th level...but, it's done.
I'll let Arthur decide how he wants to do it. We can afford it either way. Korvin could cast them all but the mag 8 spell would be much more difficult.
((You just lost a chance to bargain here. I've said, repeatedly, that specific requests needed to be made, in character. Petronius already knows some spells that could have been used. But since you are not interested... Don't expect a bargain now. This is not me reacting, it is an in-character reaction to Korvin's obvious lack of interest.))
Petronius stiffens a bit at Korvin's demanding attitude. "Very well, if it is this spell that you want..." He pores over the lab text for a bit, trying to discern its parameters, and then does some calculation. "I think this will take me two seasons to invent a ritual duplicating those effects, to create one such villa. Possibly three seasons if you want the ritual to create a group of them at one time."
"So this would be 20 pawns per season of lab work, plus a fee for each casting of the spell. Those would be of one pawn if the spell is of sixth magnitude or below, or three pawns if it is above sixth magnitude. You mentioned vis attuned to Rego Terram, yes? Half of it would have to be this kind of vis, on a one-to-one basis."
((This was just blah blahing through the current discussion of the spell that you, JL and I were having as table talk. Basically, saying that the characters were hashing out what the current spell did and what it would need to do. I did not mean to imply that we just tossed out all the other things. Korvin is interested in all options. That is why he is looking at it from this angle. His next question would be how would Petronius do it.))
delete triple post. damm server errors
((That's not the way it came off. Let's roll with it... Let's say Petronius stiffened anyway, due to a perceived insult due to botches Folk Ken roll.))
((Keep going. In character, of course. 
))