I've never lost to a wild pokemon.
To be a tad clearer:
In older games, every time all Pokemon in your team faint, you whited (or blacked) out, lost half of your money (whether from a trainer or a wild Pokemon), and appeared at a Pokemon Center.
In newer games, if you lose against a trainer, you lose the amount of money you would have won, and you lose a smaller amount if you lose to a wild Pokemon.
Oh ... I knew that you lost half your money when you lost to an older game ... but I didn't know that the amount of money lost has decreased with newer ones.
I saw my one friend lose a match when he was trying to earn the cash for a Porygon (this is AWHILE ago) in the Game Corner (remember that?). He lost to some trainer and half his money went away. Man, he was NOT happy about that one.