The simple answer: it all depends.
If it is a commonly available production folder, then the price I'm willing to pay is far lower than MSRP. If it is a rare, expensive production folder, then I'm willing to pay up to MSRP, but not more.
Customs and semi-customs are a different story. Currently, I don't own a single one, mostly due to the fact that I haven't found one yet that I find to be significantly more satisfying then a production folder.
But, when I do (its not a matter of if, anymore), then probably the limit would be about half of my short term savings. Right now, probably in the $400-$500 range, but with any luck, that number will increase as the years go by.
-- Rob