Two things I try to remind my wife of:
1. Anything that I buy that's stainless has intrinsic value. I can resell it at 80% of what I paid for it new should I decide to give up the hobby, or even make a profit if I bought used.
2. If that doesn't work, I "give up" and agree with her. "You're right. I'm probably taking this home brewing thing a bit far. Though I have been meaning to get into bass fishing. Just wish I knew someone with a bass boat..."