If the point is to thwart spammers and discourage treasure hunters, I think a fee of $10 is sufficient to work that magic. Of course if the powers-that-be think a mandatory $25 is what they need, I'll just have to give it some consideration.
Tax deductible only works if your deductions are itemized and exceed the standard deduction. Like when your medical expenses exceed 40% of income or some other IRS standard of fairness.
What's driving up prices is what people are willing to pay on Ebay. Easy to search completed auctions and see what stuff brings. You don't need this site for that. What the market will bear is a reflection on the value the buyer places on his dollar. Just like you probably think a $25 fee is nothing, where for me that's the one dinner out a week I treat my family to (Local small restaurant with reasonable prices and we often share an entrée and skip dessert.), or half a tank of gas. Paying two college tuitions makes you think twice about stuff like cable fees, local phone charges, and Internet-group surcharges!