Am just in the process of almost exactly the same thing!
We have had wall knocked down between small loo and bathroom, to make bigger bathroom. The builder's quote for this, including blocking up one window and replacing another, lowering ceiling over shower area to blend loo ceiling with sloping bathroom ceiling, and preparing the walls for tiling, painting etc. was £2320. Think there may be extras as he is also going to board the floor ready for new lino...
Not finished yet, but trying to coordinate with entire new bathroom suite, new shower and adding central heating too!
Then of course you have to add the cost of the tiles, tiler, flooring etc. electrician costs, lights, heated towel trails etc
In my experience so far, everything costs a bit more than you think, and takes a lot longer than you'd think too. Also, when knocking wall down, seems to create dust throughout whole house, so remember to shut doors to other rooms.
Hope this helps.