I think there are a lot of variables, but $50 from
each house for a 12 yr old sounds pretty reasonable to me for her responsibilities. ($7/day, roughly, per family)
My neice when she was around that age once got $20/day to "spritz an iguana"

but she lived in a very affluent neighborhood.
We paid a 12yr old (DD's friend) last week $20 to take care of our hamster when we were away. We also brought her a tee shirt and some fun snacks. She actually refused the money but DD made her take it.
We also paid a 13 yr old (DS's friend) last week $20 to water our (2) plants and (tiny) garden while we were away. We brought him some snacks also.
I've had a pet sitter for my dog in the past when he had special needs. I believe it was $20-25/day plus I gave a tip. But that was a lot of responsibility involving giving medication, cleaning and helping the dog outside, etc.
Recently my 12 yr old DD got her first real babysitting job. She took a babysitting course last year and has been a Mother's Helper, which has paid very little (difficult family circumstances, so it's essentially been volunteer work for her, and she's enjoyed it and it's given her experience, etc). Anyway, the Mom asked DD if $8/hr was enough. I told her $5/hr was plenty, seeing she's only 12 right now and she can go up as she gets older. (This was what my own children's babysitter's Mom told me when I first hired her DD.) Well a friend of mine hasn't stopped ribbing me about this ever since. My DD babysat 30+ hrs one week and he says I cut my DD's pay by 30%, and he's right, lol. But seeing as how the relationship between my kids and their babysitter turned out to be quite beneficial for everyone over the long haul, I'm not too worried about it. There'll be a greater benefit someday, I think. $8 just seems like a lot to me. Maybe I'm just old fashioned.

(Although I did feel a tad

one day when it was 95 degrees out seeing my DD pulling these kids in a wagon down the street in the blazing sun. She was hot and exhausted when she came home!)