Assuming they are just silver washington quarters, somewhere between 5X and 10X of face value unless they are in particularly good shape or have key dates included. the exact amount would depend on if this is purely a business transaction where you are trying to get as much as you can for as little as you can or if you are trying to offer a little more (or the same as) a coin dealer would offer. An offer of 10X face is actually above silver spot right now, so to offer that in my mind there would have to be something of numismatic value in there.
Personally, I would probably offer somewhere around 7X or 8X face, but that's just me.