I may be a little late in putting my view across but I would definitely say that paying for a non-biased professional to read through your work is worth it. Red Pen Proof Reading (they have a Facebook page) has been making its way around friends of mine who have been working away on their Masters and PhDs, so why not check them out? I think it's important to let anyone you hire read through and make recommendations even if you don't take them up on their suggestions!

Good luck with your work :o)