I think in general theyre popular because
1) moped folks prefer classic style of older bikes to scooters, largely because theyre "cool". And considering cool, top tanks look like classic cool motorcycles. Add to that theyre slim and the vaguely UJM/cafe style is pretty popular, even the upswept "scrambler" style is very in vogue with moto fans. Cant deny it, TSMs do have really great style. It's distinctly more popular of a style than the earlier chunkier 60's style top tanks or the later motorcycle full-fairing jives. also top tank tomos are all so gross looking somehow.
2) Theyre rare amongst cool toptanks. I think a magnum or general really looks just as good, put side by side, and personally think a sachs G3 looks even better as far as classic-styled top tank mopeds go. But those are available, most people into mopeds for more than a few years have had one or many of those. There's definitely something to be coveted of the TSM's rarity, especially considering part of the community aspect is, intentionally or not, showing off your bike, and there's credibility for finding and maintaining a rare boy.
3) Performance. Puchs have great performance options, but they're single speed (or less reliably, 2 speed). Sachs and generals are single speed. But peugeot has huge tuning potential and it's variated. Plus it's got a more stable frame and swingarm vs stepthrough peugeot making it more desirable in that regard also. still the dumb swung swinging motor subframe setup tho... Moby didnt have a real 70's toptank option like the TSM, the dealers sold the Morini'd nonvariated sebring as a top tank offering
I think that's it. top level of styl,e rarity and performance in one package. There's a few others that fit that bill, Magnum LTD gets the rarity piece that regular mag's missed, and Derbi laguna, RD50 and C5 the same. But I'd say those are all pretty equally coveted.
Regarding value, I'd pay up to 800 for a nice one, but only with the intent to resell it. They are "cool" for the above reasons, making it a GREAT candidate to sell for a profit. I'm just as happy (happier really) riding dumb crappy cheap old shit that i don't feel guilty slowly trashing, and so to me a TSM would just be a nice opportunity to get someone new into mopeds with it's sexy allure, and maybe come out ahead financially too.
Rob swoops sold this one a few years ago built up & fast, I think for $1500