Proof reading

I'm thinking of proof reading as a side job. Here's what I thought I'd offer for service, and would like feedback if it is something that people would pay for:

$20 per 1000 words
48 hours in advance notice, otherwise an extra $15 per 1000 words for a total of $35 per 1000 words
48 hour turn around time, unless needed sooner an extra $25 to total cost

English is my 1st language. Have been proofreading academic papers since 1990, mainly in the medical field.
Contract is binding through confirmed emails.

Thanks for any advice!
Janis