What is cheap? Red dots can go from $40 to $1000s....
The best bang for the buck is in my opinion is the Bushnell TRS-25, but it's around $150
Wanna go real cheap? Buy a cheap chinese red dot on aliexpress for about $25.....but don't expect much....especially with a shotgun!
As for mounting the red dot....get yourself a picatinny rail mount for $15-20 on Amazon! (they have them that will fit the Mossberg factory mounting holes)