But until you are both very comfortable in the relationship, ballpark pricing is best avoided. Clients as references. There is nothing wrong with using past or present satisfied customers as references, with a couple of conditions. One is that you have checked with the client first to be sure that he doesn't mind if you give his name as a reference. And second is to make sure that they are, in fact, satisfied.
People can be very fickle.
Take every precaution to ensure that when you cite someone as a reference, they are as happy with your work a few weeks or months after the job is complete as they were on the day you delivered it. Most will be. But for some reason I have yet to figure out, a few won't. What's worse is that they won't do you the courtesy of letting you know personally, but they'll be happy to complain to any potential client you choose to send their way. The key is simply to avoid listing as references clients with whom you even suspect a problem.