With $700, you could probably put together a rig that would get you close to 1mh/s, but even with free electricity it's going to take you forever to make that money back.

In late February, I put together a 1.5mh/s rig for about $1100. For the first few days, I was making about $10/day, but that very quickly dropped and it's more like $4/day now. I'm still nowhere close to having even half of that $1100 paid off. If you were to use $700 and put together a rig with say, 2x 270xs you would be at about 960mh/s - but, it would take you about 233 days to make that money back - and that's not factoring in any wear and tear on the equipment, and is assuming the prices aren't going to drop any further.