That seems a bit low to me. You'll have 3 trips to do the job right, right? After you pay a helper [if you have one] that doesn't leave much left over.
What is their frame of reference for that "too low" comment? Other quotes? I doubt it.
if they go to the garage to get their car serviced, ask them to check the hourly rate. I'll be yours compares nicely.

Thanks. Yes, multiple trips: two, possibly three depending on the weather.

She says she had it done before "for less", but no idea by whom or how much.

Anyway, I stopped by for a follow-up on some other work and it's becoming quite clear she's just a nickel and dimer. One week ago I gave her a quote for a crew to clean up her yard, trim branches, and pull english ivy off the house (overgrown to the second floor). Instead she decided to find folks on craigslist and guess what? It's a week later and these folks are still putzing around in the yard. The total outlay must be over my $1000 by now and it's still not finished. So there you have it.