Well besides the script, you need an actor (convincing one at least), a green screen shoot, plus the graphics, which means at least after effects unless you find stock stuff that doesn't need that much work. It could be done quickly by a one stop place-if they know and have all the elements, but it'd be expensive if they do it the "right" way. If you find a guy who needs the money and has the experience, he could get it done in a day for a few hundred bucks.