I didn't find it that difficult to fit, I watched a few youtube video's prior to fitting to get an understanding of preperation and tools required. after cutting the foam out to sit the gel insert in (flush), the seat was covered in new vinyl which was fitted with spray adhesive, before re-stappling the original seat cover back on. Cost was approx £40 all in. with about two hours labour. to look at it looks like a standard seat.
For a dealer or upholsterer to fit would have been expensive.