I could do 50,000 words for $500, which is a significantly discounted rate.
If you are truly interested in hiring native speakers who have the credentials (e.g., degrees in language studies, experience teaching writing at university level, and experience copyediting and proofreading high-stakes publications), then feel free to message me.
For the price you are offering, however, you will only get non-native speakers who have a Grammarly account, or something similar. That may be what you need though...
My bid is, of course, assuming that the manuscripts really only require a "final native touch".