As far as the price of editing goes, it all depends on what you want out of it, and what the editor promises in return for your hard-earned savings.
If you were preparing a novel for submission and wanted an expert spit-shine, you might then consider a professional editor. Especially, if the editor has a track record in your preferred genre. For example, our own Uncle Jim is a highly successful scifi-fantasy author, and he does editing for a fee. If you were preparing a novel for submission, you could do worse than hiring Jim to edit your work. Lou Aronica is another. Lou was the scifi/fantasy imprint publisher for Avon and Bantam books. He now owns the Fiction Studio. If you had a dark fantasy novel and wanted an appraisal from someone who has a couple of shelves lined with literary awards (Hugo and Locus to name two), you might consider him, or someone with equal credentials. There are other editors that frequent AW. Lisa, Dean, etc... They are good at what they do, and might enhance a new writer's chances of finding a contract.
$300.00 is an almost unheard of price for a novel-length pass/creative edit. Such an edit by a professional usually starts at $3.00 a page. Add another 25-50% for a line edit. Have you found out what she charges for essay length narratives? I'm curious, now.
As an alternative, you might find some qualified editing talent in your local English department. In the long run, only you can determine if the return is worth the initial investment. Having said that, I tend to agree with Frank (AKA Eraser as general rule, but I also believe there are exceptions.)