yeah there’s a tape input and output on the poly800 - you can save and load patch data from cassette (or just recording the audio on your computer…)
as far as prices: the polysix was like a 400-500 dollar synth not more than a couple years ago, so 850 does seem high…but that may well be the market price these days, particularly for one in great shape. i’ve seen a handful of polysixes as high as 800-850 on craigslist in recent months, but 600-700 seems to be closer to the going rate (i bought one for 600 last year, actually).
monopoly is a considerably more sophisticated synth than the polysix, so a higher price tag is likely warranted, but 1500 is very steep for one. although 1500 might be a reasonable price for an ms20 these days. oh, and don’t use any of those awakewalk prices as serious guidelines - those are boutique price tags. you should really only pay prices like that if you’re a serious collector, a lunatic, or trent reznor. haha, and don’t necessarily trust vintagesynth ratings, either!