I feel like in general, people are making their prizes way too powerful. Keep in mind the current prizes. Princess gives you +buy and -2 cost (can't be throned). It's good, but not an amazingly powerful card. Bag of gold gives a gold on top of your deck. Nice, but again not game-changing. Trusty steed and followers are great (probably worth $7 or more) but they don't absolutely break the game.
Master of ceremonies has no cost limit or nor supply limit. probably worth at least $8 (and you probably won't have terminal problems)
Craftsman pretty much allows you to gain any card and then play it. it might have diminishing returns, but prizes generally come at the end of the game so you probably won't be able to play it past its use anyway. Again, very very powerful.
Seat of honor can be better than kings court. You can TR 2 cards then top deck seat of honor to do it again next turn.
Winged sandals first option is better than champions (with the only caveat, that it's not permanent). Still way too game changing. Also combined with amulet, this can be crazy.
Prizes are supposed to be good, but they're not supposed to be crazy. Before any of you defend these cards just think; are there really that many situations that you would take a bag of gold, princess, diadem, or even trusty steed over these cards? And would they have the same effect? These cards are not only so good, they can work in almost any situation (unlike followers, which works better if there's no cursing or trashing, but not so good otherwise, or princess which is much better when there's no +buy).
P.S. Maybe it is just me. I just thought I'd put it out there.