I just bought a budget folder and am happy with my GreenZone. It was $200 with bike bag and $39 for shipping. Steel frame and six gears. The only problem is the weight load, it is rated for 250 lbs so you can't pack anything on it. However if you ride reguarly I expect you should drop some weight.
Before I got the GreenZone I looked into all of the folders (Dahon, Citizen, Bike Friday, Brompton and Tern). Most were much more expensive (some were scary expensive!) and the features were not much better than the GreenZone. My only other suggestion is look on Craigslist. I do see folders from time to time and if they were well cared for could be the budget way to go. Happy hunting!