I agree with the others. Before I even set up an appointment, I give clients a rough idea of where my prices start for the simplest quilts & I break out for them cost of materials vs. labor and explain how much time I tend to spend making a single, simple quilt (single "wholecloth" with a border, backing & binding & 2" quilting -- which I don't do anymore, but just to give them a starting point) as well as explaining that my popular applique crib quilts have an average of 10,000 1/16" needle turn hand-done stitches and machine quilting that should last a minimum of 20 years with weekly machine laundering according to the washing instructions I provide.
Once we've got that, I usually ask them to email me 3-6 photos of quilts that they like to use as a starting point for discussion. After that, I would come up with a piecing design that reflected some of the things they liked about photos they sent me and then we'd meet at the store to buy fabric. I personally prefer to be there to have input when a client is picking out fabrics, especially if they don't know anything about fabric/sewing, because otherwise they may end up picking textiles that don't play well with each other or will try to cheap out on the fabric & then complain about "my" workmanship (I've seen it happen to other quilters).
I buy batting in bulk & Kona solids (for binding) in bulk, plus the threads so clients only need to purchase fabric/notions for the top & backing unless they want a special binding (which I discourage) or silk thread. I will not substitute batting.
Be sure to calculate the cost for everything going into the quilt, plus your disposable supplies (needles, rotary blades), durable supplies (pins, markers, rulers, mats), wear & tear (sewing machine, washer/dryer) and utilities (electricity). Estimate the time for your labor & multiply by your hourly rate. If you're going to quilt and/or bind the quilt, you'll want to discuss that as well.
I have samples of the types of FMQ that I can do.
Once we agree on a design, I make a small practice sandwich that I show the client before I begin on her quilt. When she signs off on that, I collect 50% of total price (I never include cost of fabric in my price because I have clients buy that for commissioned work). In my contract, I write that if the client fails to pay, I will sell the quilt on etsy & keep the down payment & materials because people typically will pay much less for non-commissioned work, so I'm selling it at a loss. I send photos after each stage (blocks completed & on design wall, piecing completed, one section of quilting completed so they can see the progress before the whole thing is done -- although I've only had happy customers, and then I'll email them that it's completed & arrange to meet with them to exchange the quilt for the rest of the payment).
I don't really do commissioned work anymore. I had stopped completely, but I do have a close friend that asked me to make a toddler quilt for her son. I had an idea that I really liked & told her about it & she loved it. So other than the fact that she told me the she'd really love to buy one of my quilts for her little one & that she mentioned the colors of his room, it's entirely my design. I'm so much happier doing that. But I think the main thing I learned was just to always be sure to talk about costs from the very beginning, ask plenty of questions & provide the client with sketches of my design & sample sandwiches so everyone knows what's being done before it happens. And, of course, put everything in writing so you both have something to refer back to should any sort of misunderstanding happen to arise.