What version of Webi are you on?
Also, is it always months that you are using?
If you're always using months, then you'd simply want to add n-1 months to your original date.
As of 4.1 SP6, the RelativeDate function has been enhanced to include other time periods than just days.
So, your final payment date would be, assuming that your in months:
=RelativeDate([Start Date];[Term]-1;MonthPeriod)
You could then substitute in WeekPeriod if the term is weeks and create some If logic as required based on your starting data.
The RelativeDate functionality has been restricted to days since it was introduced. The closest approximation that I've got working for adding months that you might want to try was:
=RelativeDate([Start Date]; ([Term]-1)*(365.25/12))