Using Tom's numbers, lets look at cost vs energy capacity;

The old standby: Nicad 2400 MaH, 6V, $45

Energy per dollar -> ( Mah * V)/ $ = (2400 * 6)/ $45 = 320MahV/$


The challenger: -> Li-ion 3600 Mah, 7.4V, $90

(3600*7.4)/ $90 = 296 MahV/$

So, the difference in direct cost vs the total energy capacity is about 8%. Pretty good cost for leading edge technology.

Of course if you need a new charger, etc that's a consideration. Not a cost that will likely go away if you wait a year though.