Well, the thing is, for that to happen there needs to be some way to keep track of whether you spent more for a card.
Like, a card that says "you can spend more to buy this... if you do, it has extra effects when you play it" would not work as is because it'll get shuffled into your deck and you'll have no way of knowing or tracking whether you paid more for it.
So I think any later effect would have to be of the form "when you buy this, [something happens to mark how much you paid for it]". But I don't see a good way of doing that without a mat, and we know there aren't any mats.
It could be that you can just pay extra and get coin tokens, and then the card would have a way of using coin tokens.
Agreed. I see three types of cards:
Type 1
When you buy this, you may pay more. If you do, [some effect] (cards get trashed, other players get attacked, cards get remodeled, you get an extra buy, etc)
Type 2
When you play this [something happens]. You may use a token to make [something else happen].
When you buy this you may pay extra for a token(s)
And/or: when this card is in play you may buy a token
And/or: setup rules: when this card is in the kingdom player may buy tokens
And/or: when you play this, gain a token
Type 3
When you play this, gain a token, and something else
When you buy this you may spend a token to have something happen.
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What neat about all this is that it looks like there is one type of token - which means, like potion cards- there are synergies with having more than one guilds card in a kingdom.
(Earn tokens with card x, spend them with card y)
Ed