I think for all of them as 10 card stacks they're all probably getting in at around $6-$7.
Followers is a strange one, it's really powerful but it's a cursing attack that gets stronger as the game goes on. Gaining an estate really kills it as a junker, as you junk yourself, so really it's just the +2 cards, +2 vps and discard to 3 that really happens. If there's good trashing, then it's not even really +2 vps, just +2 cards and discard to 3 with deck slowdown on both sides. In that case, it's not really any better than militia. Lategame, of course, your opponents are increasingly unlikely to actually trash your curses and your estates are good, so it's much nicer. I think, considering it being slightly weaker early, that it'd be fine at $6. I'm not convinced it'd be that much better than Gold, really, if at all.
Trusty Steed has some powerful options. It's Gardens best friend, and +2 cards, +2 coin is a solid option as a $6, and +2 cards, +2 actions is better than Lab at $5. Considering it's versatility, I suspect you'd want this to cost $7 - getting lots of them isn't really that amazing, but when they collide they're still very effective (get 3 in a hand, you can still get 6 coin now and 8 silvers, for example). If it cost $6, I think they'd generally be chosen over Gold more often than not.
Diadem has to cost at least $6. As a single card it's mediocre. On many boards it's mediocre. But then you realise with good draw and +actions, many Diadems can turn a village into a cantrip with +$5 or so, which is crazy. Too much for a $5, I think. It's probably okay at $6, it'll often be as good as a Gold but occasionally better, but sometimes games will just become Diadem engine rushes.
Bag of Gold is a strange one. First time through your deck with BoG it's roughly a dead card and a Gold. Second time, a dead card and two Golds (better than having 3 silvers generally, for $6). After that, it keeps increasing your Gold quantity. I think that makes it something of a 'slow' card - sure, Gold on deck is awesome, but it's at the cost of not having a Gold now, and you're losing a little tempo to set your deck up to be even stronger in the future. I think it might be okay at $6 - generally games are quick enough that even if an early BoG is much better than a Gold, I think it'll often be ignored due to being just a little too slow compared to flat Gold. If it wasn't, well, $7 would probably work.
Princess: Princess is too powerful in combination with itself. At $6 it's probably too easy to get two Princesses together (with the extra actions), and of course three or more gets crazy. I think it kinda has to be $7, although it doesn't seem very impressive at $7.