It's been 5 years since I directed an upholstery shop, but the standard answer to that question was ~$2500 for a 4 place re-upholster (seats only) utilizing a simple design and mid-grade materials. That included new foam and overhauling (cleaning, minor-repairs, and painting) of the seat frames. Everything was negotiable from there. Cheaper vinyl, re-using existing padding, and a flexible time frame lowered that price significantly. ~$5000 for a whole interior in a comanche.
Sidewalls, carpet, plastics & trim can be done yourself pretty easily if you have the patience and tools. Seats however, to be done well, need experience to look good.
Here's a skylane we did:
I'm not sure how that pricing compared to other shops, we generally gave a generous discount for combining it with an annual inspection. Generally we were turning away customers and had long waiting lists to get on our schedule. I'm also out of that industry now, so I have no idea if material costs have changed significantly since then. I've still got my sewing machines tucked away in my garage though...lol.
I believe there a bunch of ready made type seat cover replacements you can purchase. Also some shops would allow you to send them your seat covers, and then they'd send you the new ones. We never allowed that as fitting them to the seat in a way which looks good, isn't as easy as it looks. We didn't want pictures floating around of poorly installed seats with our name on them.
I hope this helps. Others who have paid to have their comanche's done can probably chime in here. Like everything, you get what you pay for.