The cost involved in building a working sub greatly depends on the amount of work you can do yourself.
If you go out and purchase a nice GRP hull, dive module, new electronics etc. you can easily drop several hundred or even a couple of thousand pounds. On the other hand, if you have a decent set of hand tools, know how to use them and add a dollop of time and patience then the cost quickly falls, as the cost of materials is not that high.
A pressure hull can be constructed from PVC pipe, shaft seals can be purchased cheaply online, whilst the rest of the stuff you would need for a surface runner anyway.