How can I convince my mom to let me have a bday party with my friends?

Hi!! I'm turning 12 and I really want to have a party with my friends. I would invite about 8 people and have the party at my house. But the problem is my mom said it would be "too expensive" and that our house is too small and my sister didn't have a party when she was 12. But I would make a cake so we wouldn't have to buy one and I would make it Harry potter themed. And she says it would be expensive because she will have to buy the little goodie bags and stuff. How can I convince my mom to let my have a party with my friends?!?!? O and our house is small but we have a HUGE backyard. And we can't use our basement because it is full of junk.