This is the big manufacturer in China.

As you can see they are a source of many folding bikes and do a wide range of folding frame designs plus others not shown or last year's designs etc.

They make for low end brands up to Cannondale and are excellent quality.

fuji-ta.com

It's up to the importer how good a bike they give you. They will decide what components are fitted and how much profit margin they add.

After looking at various uk forums recently, some mentioning Dahon frame failures and other's mentioned that he couldn't successfully clamp the seat post without it slipping on his Dahon I'm not convinced they are that great and some of the Dahon frames actually look like they are sourced from China anyway and may even by fuji-ta (battle) bikes. I personally think either professional or user reviews are most important. The Dahon models often seem lighter but is that at the expense of strength.

I think if you are trying to find the sweet spot between quality and value you need to avoid entry level prices where too many compromises have been made but you don't need to pay excessive money.

Things I'd look for is a cassette rather than freewheel and a sealed branded bottom bracket rather than a loose bearing design. Branded hubs, double wall rims, Shimano derailleur (preferably above tourney) as much aluminium as possible but not forks. A frame with decent strengthening. I wouldn't be looking for the lightest bike just not excessively heavy. Just a hard wearing low maintenance design. Something you can buy and use and not be thinking about changing components from day one. I'd want adjustable handlebar height not just for sizing but sometimes it helps with the fold too.

Sort of bike that would not be at entry level pricing but instead at mid to high level depending on brand. I wouldn't pay for the brand though, i.e. pay a chunk of additional money just to get a more desirable brand on the Chinese frame.