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 ****-cone K's might be an easier choice to find and still kick ***. Eclipse Aluminum 10's can be found for that price. Resonant Engineering RE series drivers also fit that budget.