Rule 32C4: “In all tournaments in which prizes are based on entries, if the actual turnout is smaller than the based-on turnout, prizes must be paid at least in proportion to the turnout. In addition, if the advertised prize fund is more than $500, at least 50% of advertised prizes must be paid.”
Of course, this is for based-on entries, as you suggested. So, before you advertise the based on, consider the expenses! (Not after…)
I’m assuming you’re talking about top board cash prizes.
In your example: 32 players will equal 8 teams @$80/team for an income of $640. Subtracting the expenses you listed ($375) leaves $265. If there are no other expenses (TD fees), and no profit (other than memberships) goes to ISCA, then you can divide and distribute the remainder of $265 to each board, which will equal $66.25.
You do have to consider rating fees, however, which should be around $14.80 (with crosstables).
To directly answer your question, you are not compelled by 32C4 to award any specific $$ amount, since you did not include a $$ amount in the TLA, which says: “Cash prizes based on number of players”. No $$ amount there, so no specific amount required. If you had said, in the TLA, for example $300 based on 40, then you would have to use 32C to determine your reduction. This is not the case here, however.