The cost of overboring a motor is pretty high at most machine shops. Not uncommon for a 836 or 849
as Jerry suggests to cost $400 or more and that is with the stock sleeves. The 592, commonly referred to as a 605, overbore I generally here the same 400 or higher number quoted for the cb550. In the case of the cheap $100/120 big block cb550 motors this is as big as you can go with stock sleeves and about as big as practical. The cb750 can go bigger but as Jerry has said, it starts getting into "real money". (That's a joke, because it is some pretty good size coin just to do any size rebuild if you are changing pistons, rings, valves, springs, chains, cams, etc.)
There is a fellow on ebay a few have used who offers overbores in the $125 range and is said to do good work. Shipping will push the cost up a bit, probably a bit less than $100 more both ways, depending on box size and weight. His userid on ebay is jjhooper. He will do an overbore up to 5mm larger for $124 plus freight to match your pistons or a set he sells you.
Rebuild is extensive with most wear parts being replaced and lots of expensive machine work for boring and valve grinding, whereas, a refresh is much less intrusive and may only be a hone and re-ring, new seals, lap valves by hand, etc. So, bewary of so called rebuilt motors by others, unless there are receipts showing machine work and new parts purchases.
A refreshed motor is not bad, but unless parts like cam chains and tensioners are replaced then it is a bandaid with limited life. Anything showing wear needs replaced or you will just be back in the motor when those worn parts fail or reach end of life. Cam chains go before the main chain and tensioners for both on 750s should be replaced if mileage and wear indicate they are due. 550 doesn't have a main chain tensioner so it wears into case when it has stretched too much.
