Interesting. She doubles her expected rate, but why? Maybe she just thinks she's "worth" double what she was before, per bang. Or (more likely I suspect) she just needs or wants more money.
One clue is she flaked on you twice. She could have had $1k had she just kept both of those appointments (assuming you were game). But she flaked, getting zero from you while time marches on and her bills pile up. Now, in desperation, her hamster brain tells her she's now worth $1k per bang because, well, she needs $1k because she didn't keep her last two $500 appointments.
Don't forget, if she already has somebody who pays her $1k per bang, what does she need you for? Yet she's coming to you.
It's hard to turn these situations around. Stick to your guns and she may be offended and leave. Work patiently with her to help her understand the cost of flake. Pay her $1k and she'll see you less often. Or, maybe she'll bank your $1k and keep flaking and decide her "worth" is now $2k per bang because she earns $0 per flake while bills accumulate.
I'm really just speculating. I've never had a closely similar situation. A couple girls have meekly beseeched me for an extra $50 or $100 per meet, after several good meets, and admittedly I was giving them allowance on the low side. No problem, they proved their worth and I was happy to nudge up the PPM amount.
Let us know how it turns out.
Rockford