Planning My Spending

I am doing a psychology dissertation and may need some final language checking. I searched for dissertation proofreading cost uk, but I found many different prices and was unsure how to compare them. Does the cost usually depend on the number of words, subject, or amount of editing needed? I have a limited student budget, so I want to plan carefully. Would it make sense to check the difficult chapters myself and only get help with the final parts? How did other students decide what level of support they needed?