The cost performance ratio sucks. BUT, keep in mind SLI can be good for budget gamers.
Two mid or lower end cards can perform almost as good as a higher mid range card for less. Like gigabyte 3D-1 was a great idea, 2 6600gts for 250ish bucks. Im pretty sure it performs almost or just as good as a 6800gt for less,
Its not a waste of money. If their was no increase in performance, it would be a waste of money. Their is an increase in performance, maybe not a big one, but their is.
Hopefully sometime in the near future SLI will become more developed and whatnot, so their will be a bigger advantage, probably wont ever see a 100% inscrease but hopefully close.
Back on being mobile - ASUS G551- 15.3" 1080p - Core i7 4710HQ - GTX 860M - 16GB DDR3 1600 - 512GB Samsung 850 Pro - Steam ID: TheOtis