In reply to Heike:
It depends how much you want to move stuff around, how it's put together already and what the state of the building is under the decorative finishes. Some bits/designs are going to be a fair bit harder and more involved to get right than others. If you're re-tiling good plaster, and fitting the same size/type of suite or something very similar it's a fairly straightforward job especially if it's a second bathroom and you can afford to take your time.
I'm in the middle of doing mine at the moment, and while it's not rocket science by any stretch there can be a fair investment needed in tools and a big investment in time. That said, mine was a proper mess with two rooms needing knocking together and every single bit requiring attention from leaking drains outside, new soil stack, leaks inside, missing brickwork behind previous bodges, failed plaster, new ceiling, fan, insulation, new window reveals and sills, shower mixer valve buried in block walls, countertop and cabinet work, wall hung bog, new partition wall and door and cupboard plus various bits of damp/damage in the floor that needed sorting. Plus of course a skip.
How much it'll cost really depends on what you want. Best bet is to find parts/ideas you like, measure then sketch it up, work out what services need moving and how involved that'll be, do your research where you're not sure what material and work's involved (eg prepping a suspended floor for tiles). You'll probably revise the plan as you go to make things easier/cheaper until you have an estimate you can live with. If you can accumulate bits gradually take your time to shop around, buy it in sales, search ebay and online stores for deals. My small room: thermostatic shower over bath, wall hung bog, sink on a cabinet, mostly tiled walls and (free) refurbished wood floor is probably coming in at about £1.5k for materials including the drains/repairs etc and having shopped around quite a bit for nice stuff, never just the cheapest but certainly nothing extravagant. It'll be a bit more by the time it's heated and decorated.
jk