I need some formula help. I have the following spreadsheet created:

Spring '07 Summer' 07 Fall '07 Spring
'08
Enrollment 50 50 50
100
Programming $1500

Here's what I need:
Programming....If the Enrollment is greater than 0, plug in $1500 (but only
on the Spring column) Now, if the enrollment on Summer is greater than 0,
but the $1,500 has already been filled in on Spring, this must be 0.
Basically, there is a one-time fee of $1500 that will go in when the
enrollment is greater than 0 if no other prior semester has the $1500 on it.

Spring '07 Summer' 07 Fall '07 Spring
'08
Enrollment 0 0 225
225
Programming $1500

In the above instance, the $1500 must show up in Fall '07 b/c the enrollment
doesn't start until then and there is no sum of $1500 in Spring or Summer '07
either.

I have tried if(and) statements and If(or)'s but can't get them to work
right. Keep in mind that the figures in Enrollment can be 0's or greater
than 0's.

Thank you!!!!!