Folders can be cheap ( in terms of quality and price) and very expensive. I have a Dahon Speed P8. It's a middle-of-the-road folder. Very decent bike, nothing to write home about. Good for short rides around the city and my multi-modal commute.
I also have a Bike Friday. Expensive and fantastic road/touring bike. There is a major difference between this and the Dahon. The BF feels and rides like a full-size bike. Only it climbs better