ExtremeTech tested the performance of a low-cost SLI PC:
When Nvidia introduced us to the modern SLI system with the launch of the 7800 GTX, they suggested people could buy one card and add the second later. Or, they could buy lower cost cards, and the combo would outperform a high end, single card. Back in those days, neither option was all that practical. SLI was—and is, to some extent—a work in progress. However, it's come a long ways: Game support is more robust. More importantly, Nvidia now has a set of cards that make it seem possible to spend $400 on two cards and exceed the performance of a single $450 card..