I do disagree with your rather grim perspective on the hobby. I've been hobbying as much as you and a LOT depends on perspective, attitude, and connection has meant a LOT with my repeat girls. Your opinion is valid however, don't get me wrong.
Now, your math needs correcting. I doubt the girl averages $125/session. I would say average is $100 to $120, again, on average and at best. 12 customers/week=$1440/week or $5760/month, at the high end of this average. I doubt any of them work these consistent hours all year, so say they work 10 to 11 months at this rate. That is $57,600 to $63,360 a year. I'm certain they pay no taxes so the equivalent pay is even higher but it still doesn't break $100,000/yr. Many, of course, work more shifts too. All in all, no real gold mine, but a fantastic pay for young ladies.
To me, this does NOT mean we tip them much less, but we should also try not to overtip. That would inflate prices. Keep in mind, in the States the costs are double or higher and with typically lousy attitudes. No culture of service here in the states.