As you say it's a balance thing...I don't want the plague to be EASY to get rid of. Also, no matter what the win conditions are (ie 3 piles empty and only a couple of Estates/Duchies each) then it's still the player who has managed their plague best who wins isn't it? Doesn't really matter if the bigger VPs don't get bought in every game.
Again, the danger with Plague is that you can get to a game state where even Estates are too expensive to buy. You said that plague tokens are infinite, some diseases give out 1-2 plague tokens every turn, and you can have multiple players playing multiple regular plague-giving attacks between each of your turns. That gives a very real possibility of having everybody locked out of every VP card.
As for managing Plague, this depends on how manageable it actually is. With the concept in the OP, it would probably come down to who got lucky with lining up +Buys and getting a good disease into play rather than actual plague management. Even as is, it could be pretty swingy and come down to luck rather than good play. As you test, keep an eye on the balance and make sure that it is actually manageable.
How about this idea? When you have more than 5 plague tokens at the end of your turn gain a disease:
Disease $0* - DURATION ATTACK
When you gain this it goes straight into your play area.
At the start of your turn you may gain a plague token. If you do, carry out the attack.
At the start of your next turn pass this to the play area of the player to your left.
When another Disease is gained while this is in your play area, put this to the bottom of the Diseases pile and discard all your plague tokens.
So the question is, what is the intention behind the concept? How do you think it will play out, and how do you
want it to play out? The changes here are a pretty big departure from the previous concept.
The original Diseases were powerful effects that a player could gain to get an advantage. This version is a neutral injection of chaos into the game. It's neutral because the disease will pass around the table, and it's chaotic because it will somewhat randomly help another player clear all their plague tokens. It also has less of an impact than before because players can opt not to trigger the attack (note: less of an impact does not mean it is worse as a concept).
So what is your goal with Disease cards?
(Hospital) Well you can equate $1 to 1 copper card! Admittedly, it is not equatable to any other card, but remember that any other card may be a Victory or curse or some other useless card to you. At least the Junk Dealer gives you the guarantee of being able to spend some money after playing one or more of them. Hospital doesn't do that, and it becomes less playable the more you thin that deck.
Also, consider Herald. It's a $4+ card which potentially gives you 2 cards and 2 Actions. AND if you buy one for $5 you put one of your best cards on top of your deck.
No, I'm sticking to my guns here...$4+
In a well built deck, +1 card will usually be something much better than a Copper. Herald is a strong card, but it takes work to get to that point. If you don't have a deck with high action density, it whiffs and is just a pure cantrip, worth almost nothing. Even in an action-dense deck, it gives you far less control than an actual +2 cards, +2 actions.
When you cost a card, the most important thing (behind playtesting, of course) is to consider its function and compare it to other, official cards with the same role. Hospital is a cantrip trasher with a bonus. The family is Upgrade, Junk Dealer and Rats. Rats has more of a penalty than a bonus (it doesn't actually thin you deck), while the other two cost $5.
Compared to Upgrade, Hospital is certainly better at trashing $0 junk like Coppers, Curses and Ruins. Estate trashing is debatable, though I'd usually give it to Upgrade giving you Silver or $3 engine components (though you don't always want the available $3 cards). Hospital has added utility in sometimes helping you play more terminals. Upgrade has added utility in sometimes letting you convert good cards into better cards.
All in all, they are pretty even. And I can make similar comparisons with Junk Dealer as well.
To your specific points: in the early game, you can usually buy something after playing Hospital too. Junk Dealer has only a tiny edge there. Junk Dealer
also becomes less playable as you thin your deck, so that's not a valid point. Maybe Hospital is weaker than Junk Dealer (debatable!) but it's certainly not
so much weaker that it drops to $4. They are
really very close in strength.
(Barber) Well your wording changes the effect. Consider Processing Barber...you would not get the VP tokens for discarding plague with Barber my way, but you would with yours.
Actually, looking at it again my wording is still not very good though is it...?!
OK I'll change it to your way!
Why wouldn't you get the VP tokens with your version? You used "while this is in play" which is in effect while the card is in play, including during Procession. Perhaps what you actually intended was:
"When you discard this during clean-up, +1VP token for each Plague token discarded this turn."
For this version, the concern is whether the players can track how many tokens were discarded. It seems like something that would be easy to forget.
I prefer to get the grammar right and don't like some of the "Oh, it's common usage" excuses...so I'll check it out some time. Consider though:
"I bought one fewer CDs at the shop. I bought one more CD at the shop."
It would be "I bought one fewer CD at the shop".