Client requirement is in Budget form

Hi All,

Client requirement is in Budget form Unit rate column also needed. If any thing updated in quantity field, Budget amount field should update automaticaly those changes.

Exp:-

Unit Rate: 100, Quantity: 10 Budget Amount: 1000,

If they increase quantity from 10 to 20 budget Amount should be increase automatically 1000 To 2000.

is it possible to add one more column like Rate.

is there any relation between quantity and Budget Amount field. Can you please suggest me how we can achieve this business requirement .

Thanks

Srini