A 164: Empowered costs only two slots higher, rather than three, and provides 1.5 times the roll. Effectively, empower gives you d12's instead of d8's, and a "d9" instead of a d6. The larger the original die, the better that maximize becomes, but I don't know if it catches up to its increased cost. Breakdown: d12->"d18" avg 9.5, d10->"d15" avg 8, d8->d12 avg 6.5, d6->"d9" avg 5, d4->d6 avg 3.5, d3->d4 avg 2.5, d2->d3 avg 2.
Damn I'm too torpid a thinker. Well I hope that this is a good supplement to the ninja's answer.