
I take editing jobs, but I never quote a rate until I see the project and then I try to give an honest estimation of my time. If all they really need is proofreading for spelling and grammar I charge a lot less per-page than for writing, because the work goes a lot faster. If what they actually need is re-writing, I charge about the same as I do for original work because it takes about the same amount of time (and can take longer, depending on the specifics of the project).
The way I approach this business as a whole is this: writers are not on salary, nor are we really paid discrete mounts per-unit (we charge by the page because that's a convenient way to present the price per customer, but we adjust our price per page based on how hard the order is, etc). We are paid for our time, and it is up to us to value each project and our time accordingly. Why not take an editing job, as long as you are paid the same rate for the same amount of effort as for your writing? Why not take a math order (assuming you can do the math) as long as the client understands that the single page of calculations they receive is going to cost three times more than a single page of writing because it took three times as long to complete?
It's all about reviewing the individual project and fixing a price based on an estimate of time. This is also an advantage individual writers have over companies: we are able to actually discuss each product with a client before setting a price, whereas clients pay companies ahead of time and might overpay for something an individual writer would find quick and easy, or might underpay for a complex order that a writer could do for more compensation but that will end up getting cancelled and go uncompleted as is.