I've only seen a couple of them. Been around for awhile now. Don't know what the price would be.
I guess for short trips, where you don't burn through the battery too fast, they would be ok.
One of the hot rod magazines did an article not to long ago about the costs/advantages of adding multi-port injection. Their test vehicle got 4-6 mpg better with the multi-port. If you don't mind manuals, they always get better mpg than autos. For auto trannies, low stall torque converters help out also.
I personally think, that for fuel milleage, go with a much older, pre-emmissions model, put modern induction on it and set it up for max efficiency.
If you can find one, Chevy used to put a six-speed manual behind a Duramax. Has a really killer low first allowing you to up the rear end (or lower it depending on point of view) without losing too much in capability. Since you looking at hybrid, I would guess that towing is not really what your interested in.
If your running a small block, putting a TPI from Camaro/Trans Am/Corvette from a junk yard would probably be the cheapest route for multi-port and if you do it to an older vehicle,
http://www.moates.net/apu1-autoprom-....html?cPath=94, or something similar will allow you to turn off a lot of the EPA junk and reset the timing.