Does ammunition cost factor into this question? If it does the obvious answer is the 617. I challenge anyone to find something cheaper to shoot or as much fun.
IF you want to save on ammo AND have a Major caliber, the 610 is the best bargain. The 40 S&W is a lot cheaper to shoot than either the 38 spl. or 357 Magnum and offers more that adequate power and accuracy. In addition, if you want to step up to something harder hitting you can always bust open the piggy bank and pony up the bucks for a couple of boxes of 10mm.
Yeah, I know. The 617 costs about as much as a 686 but at a cost of about 3 cents a round those savings will add up quickly and they are a lot of fun to shoot. As for the 610, in my area 38 spl. runs about 22 dollars per box when I can find it. Meanwhile, a local Walmart has been well stocked with the 40 S&W and at 13.97 per box it's a lot cheaper to shoot. Currently I am saving about 70 dollars a month by keeping my 620 in the safe and shooting the 610.
However, if the rumors about a coming price increase at Walmart are true, my savings will drop a bit. I'll also regret not hitting my savings and buying more while it was cheap.
PS, one positive of the ammo shortage is that it indicates that there is almost Nill chance of any gun control legislation being passed. Because it indicates that a whole bunch of new shooters have joined us.