Filmy's right, you could easily find someone who would be happy to work with you for a few hundred bucks -- and you might get lucky and find someone with genuine talent and enough screenwriting skill to do the job.
However, if you're looking for someone with more of a track record, you'd need to think about two factors: time and money.
If the co-writer is taking the main bulk of the writing task, with you providing the initial idea and notes for rewrites -- you have to figure the first draft is going to take about two months of full time work. The front end deal needs to recognise that time expenditure and at least put food on the table during that work.
I can't tell you what other people would charge for that, but if someone an indie approached me with an interesting project, I'd be looking for between $6,000-$12,000 to take a script through the first draft and two subsequent passes for tweaking -- once the film was greenlighted I'd expect at least 3% of the production budget paid on first day of principle photography -- but then nothing on the back end.
On that kind of deal, I'd work as a writer for hire -- I'd release the copyright to the producer.
If the 3% on first day of principle photography was a problem, then I'd be looking for a higher figure on the back end -- on my last picture it was $36,000, to be paid at the point the film achieved distribution. LOL!
The figure is higher because the risk of not getting paid at all is higher. LMAO
I don't think I'd ever take another deal that was all back end, especially from an indie, unless I was also the producer. I don't mind risking time and effort on my own projects, but I've not much faith in other people's good intentions.
There are writers out there looking of their first credit -- and those kinds of writers do come cheaper, but I've always felt they were also a greater risk.
The other thing to remember is that regardless of the money on the table I'd never take on a writing project that didn't excite me -- unless the fee was excellent and I could be promised my name would never appear on it! LOL
Hope this helps.
By the way, if you are serious about hiring someone, I might be open to talking.