There are a lot of factors you'll need to consider in pricing your work so it's hard to give you a quick answer without more information. If you're going to set a per chart rate, you should look carefully at the type of charts you'll be auditing and what information you'll be responsible for - obviously complex inpatient stays or specialty surgeries will require more time and effort than validating simple tests or office visits. Also consider how much information and in what format you're expected to report this, e.g. just diagnosis codes, or just CPT codes - do you need to make a summary or finding or analysis of errors, or just report numbers? Are you expected to do any education sessions or presentations as part of the audit findings? All of these things can add up to a lot of time. Also, you'll need to make sure to charge not just for your time, but also for any expenses involved (e.g. travel costs and time to get to a site, printing, copying or office expenses, research materials, etc.)
If an hourly rate is possible, that might be easier than trying to come up with a per chart or flat rate if you haven't done this before, and that would help you avoid making an estimate that ends up being way too high or too low. But if you go the hourly route, be sure keep in mind that if you're being paid as a contractor, you'll have additional self-employment taxes to pay on any money that you receive for this work, so be sure to charge appropriately to cover this. Hope this may help a little at least.