All of the quotations henceforth are actual ones I have received, good or bad. If I lumped all the quotes I've heard other makers say they've gotten, this posting would be too.....colorful...
Numbr 1: put some efort into ur request
2: Provide details and expectations up front
"I have a project I'd like you to do. Please email me at .....@yahoo.com"
Unless you're Bioware or a big/awesome company that requires an NDA to be signed before any details are disclosed, chances are that any busy artist isn't going to chase you down for a project they know nothing about. Alternatively, not everyone prefers Facebook as their primary communicator, so if someone simply asks for an email address I oblige and continue the conversation there, where they usually provide details up front.
Take your car for example. If there's something wrong with your car, its your job to get it to the mechanic and give some information so they can diagnose it and give you a quote. If you can muster the strength to ask a mechanic to come to your house and peruse you for a quote, you're probably in for some pretty blank stares.
The kind of things you'll want to provide are....well, as much as you can. Commission work isn't typically an overly formal process. However, most people can agree that the information you provide before the work is started is the standard that expectations will be measured against when the project is completed. The main two things are scale factor and reference material. A scale factor can simply be a length of the object in question, or the height of the client as a basis to compare and scale specific references to. By not specifying these things, you are trusting the artist's judgement and won't have much room to complain if the final product isn't sized to your liking.
3: Don't de-value the artist's work.
"I myself have two other resin kits and those did not cost nearly as much."
"Yikes. Is that your usual pricing?"
"My friend made this out of 6 PVC pipes and some cardboard. Why is yours so expensive?"
Unfortunately we can't explain everything to everyone in regards to how prices are what they are, but rest assured no one is making pile of money here. Until you're tried it yourself, avoid sensitive remarks in regards to an artist's pricing. A simple "Thanks for the quote" or "I'll consider your offer while I get my finances in order" is a great alternative, which brings me to my next point.
4: Thank the artist/close the conversation
Hopefully I can help turn the population into clear communicators, one late-night blog post at a time!
In the meanwhile, you can harass me on Facebook, or at the site.