While I agree that it is unlikely that in the remotest forests one might
find people willing to pay 100 gold for common magical tricks, it is also
unlikely that in a city of size X, you would also only get one magician's
week of work. Notice that only one magician can do Common Magic per month
per location (not even sublocations, I suspect, but I haven't tried it).
I have a mage that knows this 'spell' and he has been unsuccessful in using
it once. As for inns being non-portable, sure, but all you need to get that
65 gold is a noble, any noble, owning the inn..he can use that 65 gold to
pay for study (after two months, say), or to give to someone else to pay
maintenance costs, or etcetera. And he gets this money with only the
construction cost (75 wood, 300 worker-hours, that's 5 months with ~10 workers
which is about 100 gold maintenance cost, so it pays itself off in two turns.
A change in the common magic scheme is probably more work than it is worth.
If you make it so that it pays less, then it should balance, such that the
first X people (X = 100/new gain) should get some. OR (perhaps a better idea)
on a day by day, week by week, or month by month) count up how many mages
used the skill, divide the proceeds among them, based upon their skill in
Common (so that a master at common magic will make a bundle where an apprentice
I never liked things that limit to the first X people (X = 1 for common magic)
who do something in a location. But I think that's a seperate issue.