I bought a 66-4 for 450 about 8 months ago. 2.5 tube. At a gun shop. They are out there just have to be patient and wait for the right gun / price. Hit the shops and learn who has finds and who has junk.
$450 is about right for a shooter in good condition. Expect box and papers to add about $100. A 4" model should go for about $50 less.
There are still law enforcement and security company trade-ins floating around online from time to time. I've seen runs of the pre-620 2002 models running ~$350 from time to time.
There were a lot of 66's made, and it's not seen as a collector's piece.
3" models? $800 might not be ridiculous, but that's a whole other story.