Something like this may be nearer the mark click here a review here click here don't know the price or the availability of cartridges you would have to check that out. Found one click here very expensive but very small and lightweight carts are pricey too click here
To be honest I think you might be wanting too much, small, cheap and cheap to run. There might be no such animal. Unless someone knows better.
This ones cheap and it certainly looks small click here have to check out the cartridge prices though or click here or click here I'm just looking at the size in the pictures and they all look rather compact. Novatech have the IP90 as well click here it appears to the only one designed to be mobile.