Should be a personal choice reflecting many factors. I think a Domme who gets intense pleasure for taking control, being more meaningful for her than the pleasure that a FS escort gets from an encounter, might want to set the donation a little lower to encourage more volume. Similarly, a client who gets something much more from a Domme than he experiences with GFE might want to offer a bit more. In my limited experience, the menu offered in BDSM is very wide ranging. I don't see why a relatively plain vanilla experience should be as high as the more complicated, equipment intensive or physically demanding of the Domme, and perhaps also distasteful to her. Personally, I will offer more for GFE because I don't get that extra "bounce" from BDSM. One person's observations.  

Totally depends.    
Equipment, planning and logistics are more complicated for BSDM.
If it's strictly BDSM and there's no sexual component, then it's more of a straight forward service.  Less taxing most likely.

If it's a combo of GF/BDSM, then I think it warrants a higher charge.  
There are a lot of moving parts, planning, higher risk to her health etc.