Tax issue

Hello
In my cart and checkout I have to put prices with and without tax.
In a scenario when my items are in net prices, I have cart summary looking like this: net price - tax rate and tax amount - gross price (grandtotalprice-price with tax). But when my items are placed in gross prices (tax included) in cart I have: gross price - tax rate and tax amount - again gross price (grandtotalprice). For me second scenario is used almost all the time, but I need to know what is nett amout of price. How Isotope counts that, or in what variabe it is located??

Thanks in advanced