It it's not your employer, then for tax purposes it will be considered that you bought your pizzas at $12.5. You always get to pay sales taxes on the $25 amount.
If it is your employer giving you the pizza, there is a different story. I can't tell you how it would work in USA, but I'll tell you how it would work in Sweden. In a few cases, which include you being an employee of United Nations, you wouldn't pay taxes on the pizza (hurray!). But if you don't qualify for the exceptions, your employer would have to account for the $25 as if it were salary and tax it accordingly. Just for amusement purposes I've computed that for you: 33% in income tax, 31.4% in payroll tax, 20% in state tax if you earn over $4500/month, and 12% in VAT (but that one your employer gets to zero in this particular example). The 31.4% is not subtracted from your payout but just paid by your employer (or yourself, if you are self-employed). All in all, you eat your free pizza worth $25 and your employer deducts from your salary $13.3 to pay in taxes, and in addition to that pays $8 in payroll taxes. That gives the tax office a neat $21 for that $25 pizza you ate for "free". Your employer also has to pay for the pizza, of course, but they can deduce whatever VAT was in its price.