There is absolutely, positively, 100% certainty that an SB will NEVER pay you back.
It won't happen.
Ever.
In fact, even if you do buy the "pay me now and bang me later" line, by the time you meet for BCD she will have completely forgotten that you already paid and will want more sugar.
She will offer one or more of these possible rationales:
1. That was favor from Daddy, I never have sex for free
2. I'm not a hooker, and I can't have sex for a measly $xxx dollars.
3. I need more $$$ to cover (insert anything like phone, rent, food, tuition/books, birth control, etc.) bill so I can keep seeing you.
4. My time and attention are special, you need to recognize that.
5. Virtually anything that she thinks sounds like a reasonable explanation.