Decimal places

I have asked this before but I think Im a bit slow cos I still dont understand how it works?

on (release){
extras = ((options.abQty25)+(options.dustQty9.50) + (options.truckQty * 25) + (options.lightQty*15));
extras.text=extras;
dayVal = day.value;
DayVal = day.value;

rate = Math.ceil(DayVal/7);
costa = rate*storesize.selectedItem.data;
total.text=costa + extras + packingSO.selectedItem.data ;
}

I want total.text to contain a number with 2 decimal places this is a lot harder than I thought it would be? I thought there would just be an option or something like that?