Judging by your figures, I'd say buy tungsten!
Been Accounting is right - I'd do something like this:
IF(B1 > 0 THEN B1 = B1 * <proportion>, {else} B1 = {cost of bulb})
.. and copy it down the range. The whole idea is that you are decrementing until you hit zero, then you bounce up again. Plotted against time, you should see eight bounces on the tungsten side for every CFL bounce.
BOFH