Well first of all, you're aware that this formula is always going to return a value of 2.50 or null?
Any session duration not equal to 5 minutes will return 2.50, any that are equal to 5 minutes will return null as they're not greater than 5 minutes (your second if clause).
As for the error you're getting, try replacing the [ & ] with #.
If that doesn't work, I'd assume your Session Duration field isn't a datetime field and I'd need more information to help you (the datatype of the field).
Regards,
Ryan.
Edited by rkrowland - 19 Jan 2012 at 4:24am