How much will it cost to build a bathroom in the bonus room?

I need to build a bathroom in our bonus room behind the garage because my mother is going to move in there and she is handicapped. The washer and dryer are in there, so there are water lines close. I have estimated by shopping online that the shower stall, doors, faucets, sink, vanity, sink faucet, and toilet will be a little less than $500, so I am just wondering what it will cost to do the plumbing and build to walls. Also, is the plumbing easy, if so i could do it myself. We wont have a bath tub, just a shower, sink, and toilet. THANKS!