it also depends on the type and color of the toning. Black toned silver coins are not particularly attractive while golden and or blue toning can be quite attractive. These days there tends to be a pretty hefty premium on toned coins because they are thought of as "original" or "untouched". However it wasn't all that long ago that you couldn't give away toned coins because everyone wanted bright white coins because they were "mint state".
Toning is nothing more than the interaction of the surface of the coin with a contaminant of some sort. I have seen toned coins where the toning was removed and I have seen white coins being toned. In both cases even experts would have trouble saying a coin was or was not original. Basically whatever the market wants is what the market will get.