Don, no disrespect intended, but at a conservative estimate averaging $3.00 a gal at 16 miles per gal you would have to drive over 4000 miles just to recoup the $800.00 cost of the cheapest efi. That's not even taking into consideration the cost of labor and additional parts.
And, you may be disappointed in the milage improvement that you may get. As Roger said above, might be better to drive and enjoy, unless you enjoy paying for improvements over driving?