There's more to the equation than cost. If you need basic developing of non-critical work, then Costco (Target, whatever) is fine. If you need more control or just want the peace of mind knowing exactly how your film is being handled, then most of us are content to pony up the few extra dollars for the DIY kits. I'd much rather be in a postion to blame myself for messing up my film than an anonymous lab tech.
Anyway, the OP was asking specifically about 120 film. If your Costco even offers that service (I doubt it; mine doesn't) chances are they send it out, there's going to be a long wait, and it's going to cost more than a few dollars.