It will be cheaper the closer it is to existing plumbing. You'll notice in most houses that bathrooms and kitchens are all next to or on top of each other. It is expensive to be installing pipes all across your house.
Aside from the plumbing and construction (walls, doors, etc) itself, it will come down to what kind of tile, fixtures, and countertop you get. We just redid our bathroom with expensive tile halfway up the walls and Silestone countertop and natural tile floor in the shower; needless to say, just the tile alone was over $8,000. Most people do not put tile halfway up the wall; it just looks better, is easier to clean (if you have a man who frequently misses, or someone who "plops" a lot during #2), and helps with resale value. Sinks, bathtubs, toilets, faucets, mirrors, and lights can all be had for relatively cheap depending on your tastes. Will you need cabinets or will you have pedestal sinks? There's a big price difference between a Formica countertop and a Silestone, granite, concrete, or Corian countertop. Some countertops, such as Corian, can have the sink built right into the countertop, all as one piece (we have this in our downstairs bathroom, but it's another brand).
The best thing to do is visit your local Home Depot, Lowes, or local business with a bathroom showroom, and see what's out there and what you like. Get an idea for what the toilet, sink(s), tub, lights, mirror(s), faucets, etc are going to cost. They should have countertops there as well. Visit a tile store and price some tile. Add all of that up, and see what you come up with. Then add on a few thousand for plumbing, construction, and electric.
Disclaimer: we used a lot of expensive materials in our new bathroom, used more tile than regular folks normally would (like I said, it goes halfway up the wall), and our bathroom is pretty big (vanity with two sinks, toilet, tub, seperate shower, cabinets, etc). If you know what a bi-level house is like, and there's usually a bathroom in the hallway, and then the master bedroom has its own bathroom, which is directly behind the hallway one. Instead of having that set up, ours is all just one big bathroom.
Oh, one more thing. For resale reasons, there should be at least one bathroom with a tub in the house. If your house already has one, and you aren't really bathtub people, it's easier to just put a shower in.
