I like this sort of thing, and use it from time to time (in certain areas of the game world). Generally moneychangers charge about 6%.
It's too much of a pain in the butt for me to recommend it as a matter of course, though. I like to use exotic coinage, though, that's for sure- either older, heavier coins (i.e. 1e/2e currency- 10 to a coin, current value depends on who's selling), exotic materials ("electrum pieces? they haven't made these in years!"), etc.