i too agree with kriag as hitting a gong at 50 yards is easy enough and I'm sure i could hit it at 100 yards, just wouldn't know why i would feel the need.
i use my shotguns for mainly home defense but i do have fun with them as well.
i also reload my slugs with a lee mold 1oz. slug as far as the cost goes if you wanted to do it cheaply as i used to you could go bye a bunch of bird shot cut the end out pour out the shot melt it down pour it into the slug mold and place the slug into the carcass of the shell you surgically removed the lead from in first place. but for the most part if you just want to plink i agree with hogdogs a .22 is always the cheaper (hard to blast the crap out of old tv's tho ) but cheaper! well thats my 2cents have fun