Post WW2 when motor assisted pedal bikes became a 'thing' in Europe as a means of transportation for many people while their countries rebuilt. Just about every province in every country had a moped "brand" where an iron works or bicycle related factory would offer a vehicle. These bikes would be assembled from a collection of manufactured frames, wheels, engines, accessories and auxiliaries, then the assembling company would brand them and sell them as their areas offering to affordably mobilize the population (often with the fanfare of buying local is the best, etc). The companies which were REALLY good at it became the brands which are recognized as the 'main stream' brands which people often debate on best vs worst.
People serving in Europe in the armed forces would often use their "perks" of being able to cargo things home inexpensively (or free sometimes) would bring home various souvenirs, occasionally it would be a moped. This was one of the methods where some of the rare bikes would end up in North America, the other being people emigrating would bring along their cycles to their new home 'overseas'.
Late 1950s-60s a few retail companies and importers saw the opportunity to bring some of the more 'main stream' cycles to North America, mostly as knock-down kits so they could save tariffs and market them as "final assembly in USA (or Canada)" and the popularity grew somewhat. The flooding of the market was however stemmed by the inexpensive offerings from Japan of motorcycles, the smaller motorcycles usually in direct competition price wise to the 50cc mopeds.
1970s came along with the greed generated 'oil crisis' and suddenly there was a surge of need for the lest thirsty mode of transportation: MOPEDS. All of a sudden they were on the fanatical side of trending, and the demand grew to the point where even the shadiest of importers could profit from importing cycles which were borderline junk and a mechanical failure within the first 100 miles (when the warranty would conveniently expire or be prorated to it was worthless). Some domestic manufacturers even set up methods to import subassemblies from various points around the globe and generate some USA branded cycles of lesser quality.
Getting into the mid 1980s the boon was done and the moped slid into decline and has slowly become the niche lifestyle which we are all enjoying.
SO - as far as 'cheap crap' mopeds are concerned, the value is in the eye of the beholder, admittedly there are some that became less desirable even while they were being sold to to poor quality or lousy aftermarket support. Some of these 'crappy' ones however are or will become an icon in their own right because they have become rare due to their crappiness. However not too many people (if any) will become millionaires rebuilding mopeds, in spite of some folks just trying to greed out and jack their prices as high as possible on eBay - Craigslist - Kijiji - or popular forums.
Pick your bikes based on what appeals to you if you are shopping for one, if the price seems high it likely is... If the deal seems good to you it likely is worth that much to you, or you would not be interested...