I've had to justify expensive (-esque, by SF standards) purchases a bunch recently as well. I'm a college student, don't have a lot of cash rolling around, but am employed. My parents and friends both shrieked when I bought $140 denim.
But hey, it was my money, and I really wanted those pants.
Especially for premium denim, there were a few things I said that seemed to help. I mentioned how well it wears, how the pants are meant to last for a long time and get more comfortable and interesting with age. They are an investment—not a pair of $35 levi's from Sears that you throw out once the crotch blows out. I anticipate them getting older with age.
My first denim purchase were Blackbird Hazelwoods (might get some gripe for that but my second pair was RRL and I still love my hazelwoods to death—I'm wearing them now!). I almost always try to mention that they're made in the U.S.A. with Cone Mills denim and what this means—high quality, and supporting business I believe in. It's not just about the use I get from the jeans—it's about what kind of economy I'm supporting in buying them. I buy from brands I like who use quality manufacturers.
That's the main thing—longevity, quality through-and-through. I'm willing to spend money on those things. I'd rather have 3 pairs of denim for my entire life than go through a cheap-o pair every 2 years because they start falling apart at the seams.