Pricing will depend on how much demolition there is to be done, where you are located, the materials that you are going to want to use.
In the past, I've heard a lot of people say double the material cost but that depends on how labor intensive the job is.
A picture would be helpful so we could know what you are working with. What all would you be replacing and what would you reuse? Would this be flooring too? What are the dimensions of the bathroom?
If you're looking for a total gut, a 5' x 8' bath with wall tear downs, additional insulation, new tile on walls and floor, new vanity, grab bars, fixtures, plumbing upgrades, etc., you could be looking at $18000-$25000 or more, if the work is contracted.
As mentioned, costs will depend on your location and what you are willing to do yourself.
There are lots of ways to save money though and diy things yourself. If you can salvage anything (tile floors, tub etc) that will help too. New toilets don't cost too much and actually help reduce your water bill, so that is usually a win/win.