The cheapest way for a quick (and short-lived) fix is some aluminum muffler tape. But I don't recommend this since it's more like a quick band-aid and not an actual FIX.

Any muffler shop can probably cut out the part where the hole is and "patch" your exhaust back together with a new section of pipe. This would be a LOT cheaper than spending money on any of the cat-back systems available.

Of course if there are more spots that look like they may rust through soon, might as well go with a cat-back. There are many options, but as far as a premade cat-back for the xterra, Gibson is the cheapest I think.

Hope this helps a little bit in your quest for repair.