Many of the cheap lasers really won't hold up over time. Also, most folks end up figuring out that lasers really have limited use on a gun. YOu can set one, but it is only going to be exactly on target at exactly one distance (it actually could be on at two distances if the conditions and set-up were ideal). A laser is bright and can give your location away as well.
Now if you run night vision and and infra red laser for identifying targets to heavy guns, then there may be a good use fo a laser. Of course these cost a bit more.
I just wouldn't waste my money on a laser, period (unless it was a toy for the dog to chase around).