Man... turn around and tell her she should be THANKFUL for your interest in homebrewing!! Seriously - for one you're saving money compared to buying commercial brews. Even if you're doing extract you're still spending a little less than half of what you'd spend on commercial beer (well.. unless you drink BMC stuff).
Then turn around and tell her she should be thankful you're not a musician. As a musician I spend waaaay more on instruments, gear, etc. Not to mention the computer recording stuff.
Then turn around and tell her she should be thankful you're not an amateur astronomer. You don't want to know how much we spend on scopes, eyepieces, filters, and other gear.
Then turn around an tell her she should be thankful you don't smoke cigars. You don't even wanna know how much money is spent on this wonderful hobby. I don't go crazy but some just spend tons and tons of money racking up their "collection".
Long story short... she should be thankful at how *little* you're spending compared to other people and other "hobbyists" out there