Depends a lot on the mission type and required endurance. To simply get up to altitude and back down again is not that big of a deal. There's a few issues to overcome but they're not too difficult in the long run. To hang around at altitude for a long period of time and be able to cover some distance with a payload is another matter altogether. Cheap does not fit into either catagory, but the former is much less costly than the latter. Small works, though.