Most people would supply and fit for a set price. If you supply the parts then how would you cover yourself with regards a guarantee? I would prefer to get the whole thing done with a firm guarantee.
Prices vary depending upon where you go and who you trust. I would only go to someone who knows how to do them and has the kit but more importantly reputation!
Russell of
http://www.mgroversolutions.com/ has quoted me £350 and that is coming to me to do it. He just repaired the wife's MGF this week and did a cracking job and that was him doing it at my house. Drop him an email via the website and see what you get back as his final price is dependant upon distance travelling from home to postcode.