Good answer, considering we know nothing of...
1. The size subs he needs
2. His musical taste
But I guess when you don't pay attention to logic, we don't neeed to constrain ourselves with minor things like that //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
12's for $100 and under can be found, but you start making compromises at that price point. eD Kx's come to mind as a very good choice, but they're hard to find on the used market. '03 boob-cone K's might be an easier choice to find and still kick ass. Eclipse Aluminum 10's can be found for that price. Resonant Engineering RE series drivers also fit that budget.