I am a dyslexia assessor and charge £500 – prices do vary but in the London area that is fairly normal. You need a Level 7 qualification and preferably an APC as well. Reports take hours to write up and the report forms used are around £2-3 each!
Incidentally the Helen Arkell Centre in Farnham have a healthy bursary fund if needed. It may be some of their assessors live more locally to you.