hi veny...
sorry for the late reply....
i don't have any knowledge about the book of Colin Drury or Horngren but i do respect DR PC tulsian .
as i had studied his books in PCC and now i am following his "OR" book which is very good for self study.
costing book for FINAL of DR PC Tulsian is also available .
question paper format for FINAL is : 45-50 marks (costing problems) + 15 marks (OR problems)+10 marks( OR theory including odd theory ) + 15-20 marks(costing odd theory)+10 marks(costing normal theory)=100marks
so you have to prepare for each of the above topic separately and in 50 marks costing problem there are many small topics in the module which you have to cover .
so prepare your strategy accordingly......
THANKS,
SUVAJIT