Back to Guilds, I noticed this text in the Dark Ages rules:
If you use Band of Misfits as a
Throne Room (from Dominion), King's Court (from Prosperity),
or Procession
To my knowledge there is currently no way to ever have KC cost less than BoM - the best you can do is make them both $0 - but this may be a small Guilds spoiler. It's not uncommon for rules to incorporate some info about future expansions. When you read the rules for the first time things sometimes don't seem to make sense. Rules may seem more complicated or confusing than need be, but as new expansions come out you can see why the original rules had to be formulated that way.
In this case we can imagine there might be a card in Guilds that looks like the following:
Discounter - Action - $4+1 Buy
Name a card. While this is in play that card costs $3 less.
As Guilds deals with paying more for cards and might otherwise modify costs, I think this might be a good fit.
Obligatory convoluted example:
- Play Spy, opponent reveals Silver, let him keep it
- Play Village
- Play Discounter, name Silver
- Play Swindler
- Give Curse
- Profit $$$$ (barring obligatory edge cases)