Unlimited. Once you’re willing to accept proxies in the first place, it feels arbitrary to say “your $1000 Library can be fake, but your $500 Underground Seas need to be real.”
Seconded. Seems real weird that this wasn't an option.
I like some cap, so that people aren't proxying commons on top of other commons.
I pretty frequently proxy random cheap cards that I just don't happen to have while playing lots of cards that cost way more. I'd hate to not be allowed to play just because I don't play Preordain in any of my Legacy decks.