What about making it a Curse card? I think overpay is the best way to make alt-Curses because only the buy is affected.
Type: Curse
Cost: $0+
Worth: -2 VP
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When you buy this, you may overpay for it. +1 VP for each $1 you overpaid.
Rule: If any card refers to a "Curse", treat it as it said "Curse card".
(In other words, treat this card as it was a Curse.)
It makes Curse games a bit longer because there are 10 more curses available (or 8/12, similar to Victory cards?). E.g., when getting attacked by a Witch, you choose which Curse you gain (if both are available).
Non-Curse games get more interesting because of the unlimited VPs.
I really don't think a second Curse pile is a good idea. It artificially makes cursing attacks more powerful. They're strong enough already. It doesn't make the "overpay for VP tokens" concept any better because nobody is ever going to buy it for $0. In fact, they will rarely even buy it at $3 because that's just an Estate in value. Yeah there are edge cases with Watchtower and trashers, but still. So we're looking at what is typically a $4 card worth 2VP that can be worth more VP with overpay... which is what the OP is already, albeit with better scaling. And that's the real question here -- is there a scaling that actually works well? That's what most of the discussion in this thread has been about.
Okay, I see that the Curse idea is almost the same as the $4 card with +1 VP per +$1 overpaid, just more favorable with trashers. (I don't consider trashers as edge cases.)
Scaling: The thing is, providing 1:1 $->VP is what Colony almost does. With the overpay, we say "the more you pay, the better the ratio." When you pay more than $22, the ratio gets better than Colony (besides getting only one dead card).
Maybe the pure "overpay for VP tokens" is a bad idea because it is unlimited, as soulnet said:
I see it too prone to stalemating, though. Why would you green at all if you have the chance to keep building money, and money is immediately translated into points? It seems to me this would be worse than the existing VP token cards in that sense. There is almost no incentive to buy any other green if any kind of big payload engine is available. At least Goons forces you to buy terminals, with this, you can keep adding cantrip money or hoarding coin tokens without any harm. It kind-of turns Baker into a cantrip VP generator, which is really bad.
I see 3 possible solutions:
a) gain VP cards instead of tokens
b) a condition for getting the tokens
c) some special effect besides the tokens
Frontier is a good concept for (a), but I also think that it does not do enough, it only adds few VP to the table and is good for three pile endings.
In the
Guilds contest, I submitted this card:
Bribe
Types: Action
Cost: $5+
+1 Action. Gain a card costing up to $4.
When you buy this, you may overpay for it. For each $5 you overpaid (rounded down), gain a Province.
The Action part could be better. To make it interesting, I wanted something that you actually want as early as possible, while being helpful for an engine, so I went with nonterminal Workshop, but meh.
The core thing is the overpay. I wanted to create an alternate path to victory - just aiming a huge amount of money without the need of +buys.
Back to VP tokens and my other two solutions.
An idea for (b), with a condition to get those tokens, could be something like "+1 VP for each $1 you overpaid, but not more than the number of differently named cards you have in play".
A card for (c), some special effect besides the tokens, would be tacking it on a nice Action, as silverspawn suggested (or as I tried to do with Bribe).
Or maybe also something exotic like "if you overpaid at least $20, you may declare the game to end after this turn".
