Like Zacs said, the price will depend on what you want. A word of warnng: insist that any builder gives you a written quotation and under no circumstances proceed without one.
Here's what to think about, so you can tell your plumber exactly what you want.
Find where your soil pipe is - where the poo goes out of the toilet and into the drains. Is it cast iron or plastic? Reconfiguring the soil pipe will be part of the job, and cast iron will need to either be removed or joined to, but either way is a pain therefore costs you more.
Where does your hot n cold water come from - the tanks. Is the hot water instant, or from a tank? If a tank, you might not get a satisfactory shower unless the location of the tank is changed, again a cost element. Knowing this in advance means you know what you're agreeing to pay for.
You could have an normal bath, basin and loo without having to alter the tank location if it's on the ground floor.
If the floor upstairs is timber floorboards, how do you want them covered - are they neat enough to not need boarding over? Will you have sheet vinyl or laminate?
Walls - all tiled, half tiled, what sort / size? Most sellers show price per sq metre but you need to factor in adhesive and labour.
Will you do any of the work yourself, or will all of it be done for you?
Will the window need obscure glazing?
Sounds a lovely project, hope you get what you want at a good price. Plus you'll gain that smashing kitchen diner space downtairs!