You should recognize that you effectively "hired a professional" - you asked a woman who works with a vet to check in on your lizard. She would probably be capable of detecting problems if there had been any problems, and, she could obviously quickly bring your lizard to the vet if necessary - luckily your lizard was apparently fine all week, but you had someone capable of handling any issues at "an advanced level" just in case.
"$20 and a cute purse" is maybe what you would give a 12-year-old who watered your favorite plant for you every day while you were away! No special skills.
For this woman, I'd say $15/day or $20/day. The purse as an extra is up to you, if you know her well enough to know if she would like it.
ETA: As MilitaryMom says, think about this woman's hourly rate at her job...even if she didn't spend an hour each day with your lizard, she spent part of an hour plus gas. Also, as Margie says, you could always do a nice gift card instead of cash. (You could put cash or gift card into an envelope and stick it inside the purse.)