Everything else being relatively equal (same market area, same caliber of provider), what should I expect to donate for a massage only vs. an escort with full service or GFE? Should it be 50%, 75%, 100%?

It should be whatever the lady in question is asking if you are talking about a reviewed FBSM provider.  As a rule I see well reviewed FBSM ladies getting about two thirds of what a GFE FS lady gets but it really can be all over the map.

I would expect there is an "upper limit" on what an FBSM provider can expect - maybe $200 or so. What I mean is that although there are many FS providers that charge $1500, there are NO FBSM provider that charge $1000.

But I do see that some Tantra providers can be well into the $300-$400 range.

According to some FBSM enthusiasts in FL,
the upper limit is 200.
Most FBSM only, well reviewed attractive ladies advertise at 100-150.

xo
TL

Guys here is what I have paid for FBSM
Tantra D.C. 300 to 600
Massage D.C. 300
Florida 250
Ohio 500 to 750 multiple hours Tantra
Escorts 300 to 500 Florida and Ohio
Its like buying a home it varies state by state
Bye the way the quality has been good in most cases