I lie to The GF and reduce the price of all my audio gear by at least 1/2, if not more. It's the only way I could own any of it without her murdering me. ^^ Unfortunately, she doesn't have any expensive hobbies, so I can't use the old trick of saying 'Well, how much did you spend on X that you don't need? That's totally comparable to what I do!' Luckily for me, she doesn't have much of an eye for electronics, so it can be a long time before she actually notices a new piece of gear--and when she does, I can just say, 'Oh, that old thing? I've had that for forever.'
I will say that I have slightly gotten her to understand this hobby by gifting her some decent listening gear over the years, all of which she really appreciates and uses frequently. My latest present to her were a pair of Audioengine A5+ speakers, which she loves to death. And when I do get new gear, she is usually interested in listening to it, and even has a couple 'test' tracks of her own that she likes to run through them. So, it's perhaps not quite an ideal situation, but it could certainly be worse! :-D