The nightguards you can buy at the store are , of course, not custom to your mouth, so unless you have the exact same mouth as the person they modelled that store-bought nightguard, it's a waste of money on a non-returnable item. Put that money toward a nightguard that will fit your mouth and wont rub when you chew or make sucking motions during sleep.

They make similarly cheap plastic mouthguards for boxers and other sports, but you have to keep them in clenched teeth to keep them in place, and be awake.