Which is exactly what I was getting at with my question.
Sometimes they actually say a $500 gift certificate with which you can buy this fabulous prize, but that's not usually what's said.
I figured with trips, it just might be vouchers to encourage the contestant to take the prize (such as from LA it's a $2000 flight, and that's the voucher you're getting, go over that when you book and you must make up the difference), and even if it's not specifically said on air that it would be a voucher, contestants would have that in the rules they sign off on prior to being on the show.
There are many prizes, especially TPIR, where it is just that, the prize. It's always been about advertising, and let's face it, how many people could do anything with a boat/trailer combo when they don't have the truck to haul it with, or any water close to home to put it on. A contestant always has the right to forfeit the prize.