Anything you find for less than $50 probably isn't worth using. You'll have to spend a minimum of $75 to get something decent. And 20mm rails are the standard. Unless you get into certain sporting/hunting rifles that have dovetail mounts and whatnot, just about everything is 20mm rail-compatible. I don't think I've even heard of an RDS that doesn't mount to 20mm rails.
It's all fun and games until someone gets hurt...and then it becomes sport!