My thoughts...I would never even bother to fill out a website form if I wanted a custom quilt done. I'd go somewhere else no matter how good the work looked. I'd much prefer to contact the quiltmaker by email, phone, or in person to discuss all the options available and work out the details. Email is great because both parties then have a record of what was said.
On your website you could just say something like 'if you'd like a custom quilt, contact me at xxxxx and we can work out the details'. I don't think there's any need at all for a disclaimer on the website about the pricing on custom quilts.
If you've done custom work in the past, you can show pictures of them in a gallery as examples in order to give potential customers an idea of what your custom work is like, but there's no need to give prices on past work that's not for sale. Just show them the quality of your work.