I have had quotes for as much as $28,000 to redo the interior of my 6-seater, and am currently redoing it for about $6,500. To get an accurate assessment of pricing, I think you have to be more specific than "redoing the interior" because there is a wide continuum of approaches (and thus prices).
For instance, on one hand, you could pull all the panels, seats, trim, carpets, headliner, and plastic, remove all the insulation, scrape all the glue, re-insulate, re-paint the entire interior or at least door jams, powdercoat all seats, toss all seat foam and rebuild, toss all old trim pieces and plastics and replace with new, toss all side panels and rebuild new ones, etc, and you could do this with premium, Italian leather and it will get you to that $28,000 number.
On the other hand, you could pull all the panels, seats, and carpet, touchup paint the seat frames, reuse the side panels removing the existing fabrics and reupholstering them, repair and paint the existing plastic trim pieces instead of replacing with new, and use a combination of fabrics and faux-leather vinyls and have the work done via a local auto upholstery shop using flame cert'ed materials, and you get to a number closer to $5,000.
Honestly, if you get a little creative and use a local auto shop, I've been really impressed with pricing. You have to do a little more legwork - you can't just fly in somewhere and hand them the keys, but pulling the trim pieces is really easy, and if you don't use new plastics, you don't have to cut and trim and re-fit everything which is a lot of work.
FWIW, I just had this conversation with my A&P yesterday. This is all owner assisted stuff - just a logbook entry that the owner can make. The only requirements are that you use flame certified materials and that you weigh everything going out and coming back in. If the weight change is negligible (which I think means within a pound, need to confirm on this point) you don't need to redo the W&B. If it is not negligible, you will need to calculate a new W&B and your A&P will need to sign that, but there are no other requirements that really apply here.