I got a bargain one time in a small town rural gunshop. I picked up a very nice 3 screw .357 for $220. Nothing more than bluing wear. It was going to be my project gun until Lipseys got Ruger to make the .44 Special. There went my project money.
But I think the average price will be somewhere near $350. Others will chime in with more but you also might look at Gunbroker for average bids.
The big expense involved with custom projects aint what the base gun cost. It is in whatever "custom" work that you farm out, and/or whatever custom parts is bought.....either/all of which is seldom, if ever, fully recovered if ya decide to sell the gun later.
Something to be cautious about with buying a "shooter grade" gun is that they not only exhibit obvious finish wear, but sometimes will have a good bit of mechanical wear as well....which sometimes aint so obvious, especialy from a photo. And if that's the case, you'll surely be paying extra to get them issues addressed....on top of whatever "custom" work is being done.
Some things are relatively easy to fix, but some can be quite costly. So ask questions, and do your homework before plunking-down your money.
T'were it me, I'd look for a good, tight gun that timed right....which probably wouldn't be the cheapest gun I could find.
OM 357's seem to go up in price perty-regularly, so even if one pays a tad too much, by this time next year, that won't have mattered.
If ya want money figures....
These days, I'd personaly expect to pay $350 or so for a mechanicaly sound example that shows a good bit of finish wear. Or more, if extra fees, shipping or taxes was invloved.....it all depends in what all figures-into the bottom line.
Sorry about the long answer. But I'm an old guy..... prone to talking too much.
Hope it helped.