Citizen folding bikes are known for being decent quality and a good buy for the money. Along the same lines is Downtube bikes
http://www.downtube.com/ss-index.html with their basic 7sp intro level bike at $299.
I'd not go much lower than these brands new -- while you can find them as cheap as the $100-200s, those bikes are junk, hard to maintain, not meant to last, heavy, department store style, with low end components. Same thing I've seen in West Marine. For just a bit more, a Citizen or Downtube is a much better bike.
Next level up ($400-700) would be Dahon and a new company called Tern. Someone mentioned a Novara from REI -- it's actually a rebadged Dahon and a good deal. At the upper end of this range, also consider a Xootr Swift -- not the smallest fold, but a good, stiff bike. After that, sky's the limit. My current folder is a R&M Birdy which I converted to 3x7 DualDrive gearing. Retail on a new bike with this setup would be something like $1600, but I got mine used for way less than half the price as a 7sp and then picked up the DD system cheap later on.
Figure out what you need. Typically, the smaller the wheel, the less comfortable range it will have, but the smaller it will fold. 20" wheels are pretty much the standard compromise; lower than that and you are sacrificing comfort and range for a smaller folded package, bigger wheels will have you riding further in comfort at the expense of storage space. There's a cult out there surrounding the UK brand Brompton, a higher (>$1200) end folder with pretty much the smallest fold in a practical bike.
Used bikes can be a bargain, but know what you're getting into. Many used folding bikes have proprietary parts which are obsolete and not easily sourced. If you don't know your way around bike mechanics, it's easy enough to learn on your own (and I'd encourage you to do so!), but if you want it checked out and tuned up at a bike shop, figure another $50-75 into the purchase price of a used bike. If you buy a new bike online, it will also likely need assembly and adjustment, so again, know what you're doing, be willing to learn, or figure some money into having a shop do it for you.
