The problem is that If you can’t prove to them that the market price is lower, they might think you are just trying to get a cheap deal out of them.
The only way I would do it which I think would have a genuine impact, is if I could send them a link to a guy which has his rates online. If that cheaper guy is well established (for e.g. has his own website), has very hot pictures and also puts up a very clearly cheaper price, you can say something like “You are really hot and I am sorry I cannot afford you. I understand that your price is your price, and I won’t negotiate you down, but just in case you don’t know this market very well, here is a site of one of my regulars with his rates:<link>.”
If he replies something like (which sometimes happened to me) “but what I offer is a much hotter service”, then you can be polite and say: “then fair enough, your price may be justified, however right now, you are still too expensive for my budget, but if I get much richer i’ll contact you again for sure”.