That is going to depend a lot on the rest of your "budget" platform. The Z-series motherboard will have to be SLI-compatible. You would need a well-made 800W (min) PSU. And a case big enough to dissipate the heat build-up of both cards at near max load when gaming. SLI is not a budget proposition, even that old of a graphics card.
The single 780 would be better suited for a 'budget' gaming rig. But even it requires a good quality 575W or better PSU. The GTX 780 is about in today's GTX 1060 class. You'll never get that kind of performance for $100. Go for the GTX 780 if you have the PSU and a CPU that can keep up.