My approach to pricing is simple: a penny per word in the original manuscript. If your manuscript has 48,943 words, you can expect to pay $489.43. If your manuscript has 892 words, I will charge you $8.92. If I send the manuscript back and you add a significant number of words (at least two percent of the original word count) before I perform another editing pass, you will also be charged for the number of words added–but the principle remains the same, and you’ll know in advance exactly what you have to pay.
NB: My rate and my method of calculating it are always subject to review and may change at any time.