Dahon has some very affordable models (depending on what country you're in). The 16" wheel models will probably be the lightest.

I love my Speed Pro -- it's a bit higher-end than the budget you're talking about, but I'm overall very pleased with the design, adjustability and quality.

www.dahon.com

There's a nice little bike-finder tool to help you sort through their various offerings.

Let us know what you decide!