Many cheap folding bikes come from the asian market (where folding bikes seem to be much more popular than in Europe). One has to consider two facts:
- Asians are usually smaller than Europeans. The frame sizes will be so as well.
- It seems to be usual in Asia to adjuste the saddle hight in a way that you can allways easily touch the ground with your feet while sitting one the saddle. In other words the seat is much to low and this position will cause knee hurts within some kilometers of riding.
Moreover (as with jour bike) the seat post angle is often quite low, resulting in the fact that the paddles are much more in front of the seat than on regular bikes.

To cut a long story short: You should definitely consider to spend much more than 300€. Quite good brands are Dahon and Tern (as said before), much more valuable (and expensive) are the Birdy from Riese&Müller or Brompton. And before spending a lot of money, you should specify what you intend to do with the folding bike. Do you want to travel with it (panier racks?)? Does it need to climb mountains? Should it fold fast and often? Do you want to make trips exceeding 20km at one time? The more expectations/requirements you have, the more money it will take, obviously. And it might be necessary to build up a custom bike. This is not as easy (and cheap) as with a regular bike due to certain mini bike specific aspects.