I have to make a presentation and in which I have to show that a separate QA departmrnt is neccassary for the company's own benefit. Now any of u can tell me how can i prove that. Since I am new to this line and have no idea about costing.
the main idea that is brewing in my mind is to take a product as example and and through tables prove that how a sperate QA dept. would be better than just programmers doing some sotrt of unit testing.
I don't think you can do it directly.
First, you could analyze some old applications, like you want to, analyze the founded bugs, try to figure out want kind of test would have discovered them (negative, boundary, scenario,...) and it what stage of the product. Evaluate the cost of poor quality (looses caused by this bugs, costs to fix them,...). Then evaluate the costs of having a QA department, the benefits obtain from here (in money, if you want to convince your management). You could add some other non monetary benefits, as company image, customer and employee satisfaction, ...
Anyway, it's not an easy job, and I see no easy way to do it.
Good luck !
... Not even me.