You only buy the knockoff because your income is constrained. If your income was higher, you would be buying the brand that was advertised to you. Knockoffs are inferior goods, which is a Economics term for something that you buy less of when you have more money.

The ad worked on you because you want that type of item instead of something completely different. For example, an iPhone ad will not just advertise Apple but also smartphones in general. When you see the ad, you are enticed to buy an iPhone instead of an Android. But also you are enticed to buy smartphones in general, instead of a snowmobile.