If it's only a crack, you must be getting some (or a lot, depending on how damaging the cracks are!) "squeeky" noise specially when you hit the bumps on the road. If the mounts are completely broken your car will shake like you are experiencing an earth quake even when it's idle -- the mounts hold the engine in a steady position when it's running. Shaky engine can also damage other parts of your car, which will eventually cost you a lot more. It's better to change it if you can afford it.

Cost: $100-200 at a junk yard, and perhaps towards $400-500 at the dealers'.