This was discussed recently in the "over unity" megathread, and I think a reasonable low-cost (and reasonably green) option is a propane powered generator that can be used to recharge the EV's battery pack overnight or during an 8-hour work shift where other charging sources are unavailable. A 4 kW generator is about $400 and will run for eight hours or longer on a 20 pound propane tank. That will give you 32 kW-h of charge or over 100 miles of range, only adds about 150 pounds, and may fit in a good size trunk or a small trailer. A little portable 800 watt generator will give 6 kW-h in 8 hours, or 18-24 miles of range.