Final Date Question :-)

Hi there everyone, OK here is a final date question. It's complex (Atleast to me :-( but i'm totally stumped. Here's what I need to do. Say September is Summer and October is winter, now getting the dates for each month is easy. Now, say in September each day is worth $50 but in October each day of the month is worth $70. Now finally, say you choose 2 days in September and 2 in October (And this is what stumps the living crap out of me - pardon les francais :-) the system has to know "He chose 2 in september so that's $100 in total, but hold on, wait a minute, he chose 2 in october and these are worth $140 for 2 days) - so how can I get it to do that? I know how to get it to do the september, but finding that it ran into October and that Octobers 2 days are worth more than septembers just totally goes over my head :-( I don't know how else to explain it so sorry if I confused you. Chris