I would say it depends on what you plan to carry. While Burley's are expensive, you can often find them pretty cheap used. The plus side is it'll somewhat protect what's in it from the elements.

I don't personally have experience with 'cheap' flatbed trailers, but I agree with [MENTION=326911]osco53[/MENTION] and [MENTION=11263]Nightshade[/MENTION], about considering building your own (I'd personally use bamboo, as it's light, strong, cheap, easy to work with, and easily repairable), if you're at all handy.

There are some pretty cool examples here: The Bamboo bicycle trailer & DIY bicycle trailers | MAKE

Keep us posted!