Thanks everyone! But is there any way to say the meaning that you spend money to buy A, in return, you can get A + B, and show that B is bigger?
I make up one "Buy a Big Deal"
Is this expression sensible?
No, it isn't.
The "free gift with purchase" is not normally "bigger" than the original item purchased.
I could see buying a coat and getting free gloves. Or buying a chainsaw and getting a case for it for free.
But not buying a set of tires and getting a free car.