Regular pedicabs are expensive.
They have had some cheapo pedicabs on Ebay that were fairly cheap. They're single speed, single passenger seat. From what I've read, they come VERY unassembled (as in, you may have to lace the wheels, I don't know).
Look at some of Worksman's trikes. They're not cheap, but still cheaper than regular pedicabs. In particular, the "Chariots".
www.worksmancycles.com.
I think there are one or two bike trailers that can be used for adult-sized people as well.
Occasionally, you'll see old asian pedicabs for sale. They normally are single speed, go for maybe $300-$600 depending on location. And there's no telling what kinds of parts you'd need to fix one.