To the OP
/edit - this was posted while I was composing:
660s will typically be in the $500.00+ range, depending on condition.
066s should be in the $500.00 or less range unless mint/excellent condition or complete rebuilds.
Right now way over $500 on the average.
For the pulse of the market check ebay completed listings on what your buying. Even non running parts saws are near $500.
If you have to stick to $500 be prepared to wait till after Christmas. Right now used prices are approaching record levels.
I've been tracking COMPLETED listings of Stihl 066/660 sales on ebay off and on for over a year now. Last week the average price, not including the ones listed as "new", was about $720 w/ shipping. And note, some peeps sell the saws with FREE shipping, some charge up to $100.
I bought this freshly rebuilt powerhead for $515 last January.
Last December prices were in the $600+ range. Prices crept down a little more after January.
Keep an eye out for CL and estate sales, but they tend to track ebay prices.
If you go for a 660 don't pay much extra for a muffler modded saw, they aren't worth it. You can do it yourself with a Dremel or for $50 buy a DP cover from Bailey's. And be wary of claims of "woods ported". If that's properly done it's worth $$. However I think some peeps drop the muffler, clean up the exhaust and call it "woods ported".
I've got both and they are LOUD and PROUD.
I don't know dip about huskies, but the 660 with it's inboard clutch and side chain tensioner also makes an ideal lower end milling saw. Open it up w/ a DP muffler, get er' snellerized and your approaching 880 stock power.