Believe it or not, the API Tap Water Purifier works pretty well. It's a DI type unit, not RO, but I tested the TDS out of one once and it was pretty close to zero.
It's not an RO or RO/DI but if you want uber-cheap that works half-decently - that's it.
OR something like a Kent 10gpd Bare Bones RO. It does not have DI, and it will take 24 hours to produce 10 gallons of product water (along with waste water) but it works well too.
I may still have a used one kicking around here - it would need a new membrane and carbon cartridge as it is older and I know it hadn't been serviced when it came to me, but if you're interested in that, shoot me a PM and I'll get you more info on it. (And make sure the one I think I have is still here)...
Jenn
Jenn