Create a Calculation field with the following calculation (with the result of the calculation set to Number):
Case ( Get ( RecordNumber ) = 1 ; your_field ; your_field - GetNthRecord ( your_field ; Get ( RecordNumber ) - 1 ) )
For the first record, the calculation will return the value of your_field. For all following records, the calculation will return the difference between your_field in the current record and your_field in the previous record.
Thanks, works perfect.
Just be very careful with these kinds of calculations to keep control of your found set and sort order or you could get results that are not at all what you needed.