cost per db

ok so i recently built some bass cabs that double as upper bass depending on the show im doing. ok so im happy with the results but i know for lower use i need to use stronger drivers. OK aside from amplifier cost, is 2.6x the power and 3x the cost per driver realy worth it for 5db @ 50HZ?

ROI?
well heres the deal, when we do big shows my buddy who runs his own audio company and myself team up, hes got dual 18s for bass and mine are used for kicks, his tops, and mine for monitors. regularly i do smaller shows which i dont have room for alot of bins. right now im reasonably satisfied with the sound but i do wish i had a bit more in the lower end of things. if i cough up the 300 a driver for 4 drivers at the same power i would actualy get a bit less sound acording to win isd. however with alot more juice (1200w VS 450W) i can get 5db more at 50hz and 2 more 120-200, but that also will require me to get an amp that will do 1200 a side to run them.