I have found 6 to 10% normal depending on the State and what you ask them to do. But the one's I've worked with or have contacted might work with, all have a minimum and your rents wouldn't cover it. One is 8% with a minimum rent of I think it was $500 or $600. I don't have any under $900 so I forgot the actual number. Another is 6% because of the number of properties, but with them I have no rents under $1,500 and all have nice schools ie neighborhoods.
As @Jose Guevarra mentioned, I have found monthly flat rate PMs but after talking to them I found them to be a whole different class of PM. They were more of a freelance employee. Very little provided, much more than a phone number for tenants and minimal support.
Separate tenant placement fee from operational PM. Same office, maybe same person but a different service. Some are a half to a whole month rent with a minimum. Some charge a fee again when a lease is up, maybe 1/2 the original placement fee.