Few years ago now, but it cost less than 8k after purchase of the boat. Boat had not been sunk but there was water under the floor. I stripped the entire thing back to an empty shell. Made a new self draining deck myself and fully welded the rear bulkhead.
Blacked it and welded a couple of plates to the base where the wear lip was getting a bit thin.
The inside was sanded down and pained in red oxide. Battens were all off cuts free from the wood suppliers skip, Insulation and oak faced ply were the most expensive purchases.
Engine was fine but I lifted it out and cleaned it up. Decent second hand engines can be had for a good price on eBay or Apollo duck if needed.
Windows out, boat sanded down and painted it to a respectable finish, reseal windows.
All in all I got a boat that I knew every inch of, took about 18months evenings and weekends but I was in no rush. I printed all the pictures of every stage of the restoration and left them in a file for the new owner when I sold up.
If you can do all the work your self, a profit can be made even on an older boat. It's easy to say go and spend £30,000 plus on a new shell but when you don't have that sort of money to go at you make good what you can afford.
And stuff you can get cheap from neighbors who are upgrading. I got a nice wood burner for next to nothing from someone who was having a new one fitted.