I've had good massages (and I'm familiar enough about massage techniques to be able to tell a professionally-trained one from an in-house-trained one) at Candle T Spa, sanctuary, etc. They usually cost something close to 1,000 baht for 90 minutes, so I'm pretty sure the therapist is well-compensated from that price. Naturally, it being Thailand some tip is still expected if nothing else, at least a token to indicate a gesture of thanks. As far as I recall, at these places, I'm always at least moderately satisfied - they are quite consistently good - and so I tip at least 100 baht. If I'm truly happy with the work, I tip about 200-300 baht.