Cost: Between $2 and $10, depending on how fancy you get
Time: 10-20 minutes
How about a door stop, a couple magnets and some glue. That's really easy, really cheap, and actually can be pretty stylish! These magnetic feet can be crafted quickly, and popped on and off when you need them. This also works with things that just prop up the back; for instance, we've seen a couple wine corks do the trick just fine.