Recently had a custom retrim done on my boat, for one colour leather, and different colour piping, with stitching. It worked out at about £70 per seat, and £130 for the rear bench seat and headrests.
Side cards on the boat are bigger than a car and they were £120 for a pair.
This was including the custom sculpting and moulding of the support foam and the design of the leather/piping.
Interior retrims really aren't as much as some people would think and I think they are a great way to personalise something! Perhaps look into it?
I got several quotes from "professional" interior companies and one of them wanted as much as £3k to do the work! Others were about the £1k mark.
I would highly recommend going down your local high street and asking at the fabric type shops, independent ones, that's the kind of place I got mine done. Only took 3 days including patterning and fitting!
Hope that helps,
Alex