Based on your base (so to speak), I’d create a second formula field for each month that would represent gross income. The formula would be something along the lines of
{PreviousMonthGross}-{PreviousMonthNet}+({ThisMonthHours}*{HourlyPay})
The formula for {ThisMonthNet}
would be
MIN({ThisMonthGross},2000)
That’s assuming you can do all your math within each record — that is, each record contains multiple months’ worth of data for an individual. If you need to perform cross-record calculations, it’s still possible… just more difficult.