I bought a Mercier Nano from BD a few years ago, and I can tell you it’s a rubbish as the $300 price tag suggested. The frame is like gas pipe (i.e. heavy and dead feeling), the components are bottom of the barrel (e.g. non-sealed headset...JIS, too, just to make replacing more difficult!), and the build and assembly quality is very low (e.g. insufficiently greased bearings, gritchy hubs).

I’m 6’ and got the large frame, 55cm IIRC. I had to buy a new post to get proper leg extension, and new stem for decent bar height and extension. Bar diameter is the old— what?— 24.5mm standard, another thing to make finding replacements a bit more difficult.

After rebuilding the bike with proper lubing and adjustment, it rode a lot better. I also replaced the extraordinarily crappy brakeset with Tektro units (long reach are required), ditched the small, stamped steel chainrings for bigger (60t) alloy units, and replaced the cassette to get as many GIs as possible, because the stock drivetrain was stupidly undergeared for my needs. New tires and tubes; the stock rubber rim strip kept shifting and exposing the nipple heads, causing flats, so I properly taped the rims.

As it is, it’s rideable, but barely, and I keep it for the fun and shock value more than anything else. It’s fun to break out for quick trips around town, or to run short errands, and for bike festivals and stuff like that. It’s far too junky for sport riding; it does wheelie like a freak, though! Handling is lousy because I just smother the whole bike under my hulking mass, but it did better with Schwalbe Kojac slicks than the wider Tioga Powerblock BMX tires, which were just too flexible in the sidewall, despite yielding comfort gains.

In the end, as I recall, I spent the same on mods as I did for the whole bike, so to the question of whether the more expensive ones are better than the cheap ones, probably yeah. Oh, and definotely pay attention to geometry, because a lot of these are just small, and have little consideration for proportional sizing.