Since you received a cash prize, the amount received is also the Fair Market Value (FMV). TurboTax asks if there is any adjustment to FMV.
If you entered prize winnings from a 1099-MISC in TurboTax, once entered you will see a screen titled "Fair Market Value of Prizes and Awards". There will be a box next to Adjustment to Value of Prizes and Awards. Having received cash, you will leave this box blank. (If you had received an item, there may be a difference between the FMV of the item and what was reported to you.)
See screen shot below.
To get back to the "Fair Market Value of Prizes and Awards" screen:
Once in return: Federal Taxes -> Wages & Income -> Other Common Income -> Revisit Form 1099-MISC -> Click Done on "1099-MISC Summary" screen