Another option:

This often works well but each situation is different, your mileage may vary, etc.......

Get a nut of about the right size for the broken bolt. Weld it onto the stub (maybe just the end will be visible) of the broken bolt. WHILE IT'S STILL HOT back out the bolt. If you don't have access to welding gear have a mobile welder come out. It'll still be quicker and cheaper. Be sure the car is up high enough for the welder to get in there and make sure you have a reasonably clean and dry surface for him/her to work off of - it'll be cheaper.

Dan