I wouldnt say its cheaper...i would say its more affordable.
After your start up costs (press, dies, powder measure, powder scale, tumbler, media etc etc etc) then you have componet costs i.e it seems every caliber and every weight of bullet requires a different powder.
All that being typed, once you dial in what your particular gun likes to eat/you shoot best. Its then MUCH less expensive per unit cost. Also, when you decide you want to go shooting, just go to the reloading room, throw out a couple 100 rds and not worry about stopping at wally world and worrying if they are gonna have your particular round in stock.