You are supposed to reduce the value of the deduction claimed by the fair market value of anything in return. So if they send you a $10 sticker or invite you to

You are supposed to reduce the value of the deduction claimed by the fair market value of anything in return. So if they send you a $10 sticker or invite you to a banquet with $500 in food, you can't deduct that part. The question is, how exactly can one determine the FMV of a boost in admission consideration (not even just the admission itself, but an un-quantifiable additional favorable factor in review)?