^^What MMUK said^^ adjusted for your area, tastes and expectations. Also, you need to be clear about what you're asking - not just here, but when you're getting the work done! I think we're assuming here that you don't want to keep the original (downstairs) as a second bathroom once you have one upstairs, so what will the original (downstairs) bathroom become, as this will affect the cost of the making good e.g. if it's a utility/laundry room, then you may be able to live with the tiled walls; if not, then they'll have to be taken off and re-plastered...
FWIW here in London my 'ball-park' cost for a basic bathroom refit is £4k - 5k, plus all hardware, sanitaryware and consumables. Electric under-floor heating adds about a grand to a small bathroom. This is all done with qualified sparkles who will certificate their work, btw, and my plumber's also gas safe (was CORGI) certified.
A basic white bathroom suite can be had for a few hundred quid, or you can spend a few thousand; ditto with taps, towel rads, shower pumps etc.. etc..
HTH Pete