The JL box, while undoubtedly far superior, is 3 times the cost just for the box. Then you add an amp, and the wiring and your probably at 4 times the cost.
In addition to the cost you have to consider the rest of the system. Ince you add 2 12's with decent power what do you do about the highs? Your either going to have to turn the JL way down or start replacing door/dash speakers and adding amps to power them.