There are lots of variables in this equation. For one thing the style and grade of construction is important to cost, not just for the conversion but also the new garage. A simple 2-car garage can be as cheap as $5,000, but most people realize that durability and quality of construction demand more material cost than that price will allow to be paid for.
The distance from the plumbing to the bathroom you're adding and the pathway it takes is a critical consideration. If, for example, an existing bathroom with hot, cold and drain lines is on an adjacent wall and the floor of the bathroom is at a grade that can pass the lines without added expense, then the new bath won't be more than a few thousand. New insulation, sheetrock, electrical and so on is fairly straightforward to install, making the living area also only a few thousand. You might be able to get away with a window/wall A/C and space heater, but upgrading the existing furnace/evaporater/compressor and air handler plus the ducting to make it work, could set you back several thousand.
However, there are always complications, and if the plumbing is not close enough to the new bath area, look out. You need to consult a contractor to get a better idea after you have some ideas about what you want. He can help you design the living space and plumbing to be more economically installed. It sounds like you should expect to set aside at least $20k for this project. But if you contribute labor to it, the amount could be less, or if the esthetics and quality outweigh price, then it could be more.