I've been tipped as low as 10 bucks and as high as a damn SUV. If her service was great and you think she EARNED a little extra, go for it. If it was just the bare min. of what she had to do, whatever.

(I'm presuming that since you are using the term FBSM as opposed to TM, that you mean a HE in addition to the massage.)

For example, if she works at a MP where the rent is very low, say $50, then you know she is getting nothing from the house and anything she collects from the client as extra is what she makes.  In that case a "tip" of $100-150 is not out of line depending on various factors.  For example, did she disrobe at all?  Did she try and prolong the experience and act all sexy and turned on?  Of course the massage itself is something to take into consideration if she really gave a very good massage.

I've had some FBSM experiences that were much better than some FS/GFE experiences on account of things like that.  Those gals earned everything and then some.

On the other hand, if she is an independent FBSM gal charging a decent fee of $200-300, then there is no real need to tip again, unless she did some things that were clearly above and beyond the call of duty.  I'd say $50-100 depending upon what those things were.

You should just see a Legit Massage Therapist for just a Massage.