Eloping Advice!

We are wondering when and how to tell my parents? His would be completely fine with it! 

My mom is very controlling and materialistic. My brother is currently planning a wedding and his Fiancé's family has a budget of less than $4,000 for the entire affair. My mom says that is unacceptable and is now throwing thousands of dollars into this wedding so that it looks good, against my FSIL wishes. She is making their wedding all about her and I can't handle that for my own! She also has said that I now have to wait a few years after theirs so she can afford my wedding and theirs isn't for another 2 years!! They have only been together for 1 year and were engaged before their anniversary! My Fiancé and I are going on 6 years together and 2 years ago my mom told him that we weren't ready to get married and he couldn't propose- and now all of this. I just can't! 

I have never wanted a big wedding anyways as I feel that a wedding is just for the guests! I could never be happy spending that much on one day or on a dress I'm going to wear once for maybe a few hours!! The whole thing just is not in my best interest and we both agree that eloping would make us so much happier and a more intimate thing! 

How do I tell them? I know she is going to say that I "robbed" her of the day and that I am being inconsiderate of her feelings when the day isn't even about her.

We agreed not to tell her beforehand, because we don't want the day or week ruined. 

Thanks!