try this (works in Excel 2002PL):

D20=His monthly cost (234.25), F20=Hers monthly cost (234.25)
D22=His current age in years (59), F22=Hers current age in years (59)


=(79-D22)*(12*D20-MIN(12*D20*25%;500)) +
(83-F22)*(12*F20-MIN(12*F20*25%;500))

--
==============================
Janusz Pawlinka