Autogeneration of id in gridview

Question

Hi,
  I wanted to generate a purchase order number in gridview serially just like cbu001,cbu002,......The user will click on add new row button then new row will add to gridview with Po No. like cbu001 in purcahse order No column. if again user press on add new row column means again cbu002 should generate after entering all the fields in the row the data should store in database. I am not getting how to generate this id inside the gridview please anybody help me.