Well, I think any can be in the regular Supply, because they all have their uses.
The hard part is giving them the right price.
Trusty Steed is definitely better than a Lab, but if you make it $6 you can compare it to a GM and GM even has the buy restriction. So is it better than GM? I don't think so, maybe $6 is a good price.
Followers is a little hard to gauge, sure the attack is brutal, but it hampers you as well. Sure would be funny to have it in the Supply with Baron. It's a Torturer with both options applied, so maybe $6 here?
Bag of Gold, if we make it too cheap, it will be too easy to get Golds. So should we make it as expensive as or even more expansive than Gold? Maybe so. $7 here? The +1 Action makes it especially juicy, since you can always play it.
Diadem, this is a card that should be handled with care, since it can be insane if a deck is built around it (Fishing Villages for example) or worthless in decks with few +Actions. I don't see this costing more than $5, maybe even $4. It's hard to get $3 out of it as you would with a Gold.
Princess, the obvious comparison is Bridge, but this can't be TR'd or KC'd and Bridge can, which makes it worth just a little more. Bridge costs $4, so I think $5 is alright.
Pricing cards is somewhat new to me, so I'm going by feel; if you think some are way off, please say so and more importantly, tell us why.
We have to keep in mind why these cards became prizes, maybe it was because they couldn't be priced right...
Donald says:
The Prizes wanted to be cards that I wasn't "wasting" as Prizes. Cards that I couldn't do normally, because they were too hard to price well, or were too powerful in multiples, or too narrow. You don't always have extra actions for Diadem, but you can just take another Prize instead; it's not a whole unused pile.