I just had mine done at SMB texas. Including bed it was just under $6k. There are not many options out there that I could find. Colorado campers (
www.coloradocampervan.com) near Fort Collins does what looks like a nice job for less, but currently only modifies conversion van tops that are typically 8 inches or more higher than the stock top. SMB only adds a couple of inches.
The CCV owner has a thread here somewhere on one of his builds.