regarding rates for freelance copyediting

This may have been asked here before, but I don't recall seeing a recent post about it. I'm an amateur copyeditor, about to offer my services on a professional freelance basis for the first time. To be specific, I've been asked to quote my rate for copyediting academic papers in the medical research field.

Would this normally be priced in the hourly range, or by the page? It's copyediting, straight-up: no checking of content or references. The first paper I was asked to quote on was only 5 pages long including endnotes and took me less than 30 minutes to edit. But I feel like some sort of a minimum charge would be reasonable; I'm just not sure what's reasonable. Is $25/hr a reasonable-sounding basis on which to provide a price quote for these services? (Having said that, $12.50 for copyediting anything seems a bit gauche; would a minimum charge of $25 per job seem reasonable, maybe?)

As you can see, I'm a neophyte at this, inasmuch as copyediting is concerned. Any advice or insights warmly appreciated by me. And geez, I sure hope I haven't made any typos in this post.

(x-posted to cranky_editors)