the difference in VV mods is the expensive ones have a computer chip that "regulates" the power so that you get the same voltage that you set each and every time, throughout the discharge cycle of the battery.
If i couldn't absolutely couldn't afford a ProVari (or Darwin), I'd look at the Infinity, which does have regulated variable power; it just isn't made as well (durable), doesn't have readouts of various info (battery charge, resistance specs, voltage,..) ... it has a little wheel for setting voltage. About 1/2 the price of a Vari/Darwin.