We redid ours about 4 or 5 years ago, I think it was around $ 3,500. BUT we used an upholsterer who was just beginning to work in the marine industry and was well below the market, cost wise. She did a great job but disappeared after the job was done. We were able to reuse the foam in the seat sections which also saved us money. We had to replace the foam in the backs though.
We kept the bull nosed seat cushions but some people switch to a flat cushion, which is cheaper.
One of the nice things about the 400 is the huge expanse of cushioned seating on the FB. It's not so nice though when you have to replace it. It would not shock me to get an estimate for a complete job, with all new foam and bull noses, to come in close to $10,000.
After we got our new cushions I started storing them in the guest cabin when we are off of the boat. It takes a little while to get them up and down and they take up a lot of space in the cabin, but they are so expensive that it's worth it for me. They are holding up much better than our old ones did which were left up in the sun and heat. Our cushions are pretty much like brand new after 4 or 5 years of use.