Looking to calculate months

Hello,
I have a simple spreadsheet that will calculate rental payments based on any given month for 1 month.
I'm looking for help that will calculate based on 2 or more payments.

I'm using this to calculate 1 month - =IF(C2="","",(DATE(YEAR(C2),MONTH(C2)+1,0)))
C2 would equal the 1st day of the month (11/01/2012), the formula would return 11/30/2012.

If a customer wants to pay for 2 months, I would change the formula to calculate 2 months.

I'm trying to get away from having to change the formula when someone wants to pay for more than 1 month.

See the attached.

Any ideas?
Thanx.