With all due respect, and maybe I misunderstood something, but I don't believe that having operating expenses is reason enough for tipping. Every business is expected to include their expenses (gas, rent, laundry, supplies, etc) into the final sale price of their goods or services. It may be just me, but when I pay for something, I'm aware that just a percentage of that money is profit, not the whole amount.
A tip is something you give for above average service. In my experience, I tip an MA on this situations:
If the activities during the session went beyond the standard sf2, massage, bodyslide and HJ.
If she did the first move and included something beyond the activities mentioned above (like lfk for example)
If we just had a basic session, but she was able to make it feel as natural as possible (great attitude, relaxed, conversation, etc) maybe we went above the reserved time a bit.
If more activities are offered beyond the regular MA session, that's no tip IMHO, that's add-ons to the session, so I don't tip on top of that.