Using data from ComboBox and DataGrid

Hi there!

I have a ComboBox (CBBUY) and a DataGrid (DGBUY).
In the ComboBox, user choose Items Issues of a magazine).
In the DataGrid appear as Objects all the chosen Items on three columns:
Item number (nber_dgc), kind (kind_dgc) and quantity (qty_dgc), default value = 1 (qty_dgc is editable so user can modify it).
kind_dgc commands the prices to pay as there is single and double Items.


How do I pull out the data contained in DGBUY with updated quantities and then calculate for each row the price to pay depending of the kind_dgc parameter?
Should I turn Objects in DGBUY to Strings?
And how I extract the quantity from each Object in DGBUY?

In the future:
I wish to use a DataGrid with CellRenderer.
Where can I find a GOOD Tutorial for this task?

Thank you very much in advance for your answers!

Best regards,