Can you please help me with buying a LEGO set?

I have wanted to buy a LEGO Technic 8275 Motorized Bulldozer for almost 3 years now. I don't know how to tell my parents I want to buy it. It is discontinued, and now it is 400 freakin' dollars on Amazon! I REALLY want it buy I don't know how to tell them. My parents never let me spend my own money and I saved enough to buy it. They treat my money as "family money" and that I am spending my parents money even though it is mine.