For 800 bucks (assuming it's a 4" barreled 66) it'd have to be in LNIB condition -- for me, anyway (only because I've already got a nice 4-incher). Supica's and Fjestad's 2021 guides generally reflect 500-700 for one in excellent condition, with NIB up to 900, add 10% for the 2 1/2 barrel. All these used-gun value books seem to reflect way lower than what I see used guns go for in my region, the PNW. I'm seeing 66s (no dash through dash-4) in somewhat suspect condition going for a grand or more these days, so if it checks out good and it's a clean specimen, 800 is definitely in the realm of possibility if it's affordable to you.
Personally, the Model 19 and the Model 66 are at the top (for me, anyway) of all the .357s ever produced...