I'm typically the girl buying whatever's on sale when it comes to cosmetics and whatnot- shampoo + conditioner in bulk and on sale, blush / eye anything on clearance and from a drug-store, etc... However, there are two things that I really love and end-up splurging on:

1)
Some kind-a gorgeous foundation from benefit.
I'm not going to rant and rave as though their paying me to do this, but... it's good stuff.

2)
Head Rush by bed head- "shine adrenaline"
I, like most girls, wasn't blessed with that perfectly straight hair or gorgeous curls- I survive by way of my flat-iron, and realized shortly after discovering it that my hair breaks pretty easily with the help of that heat. This spray is light enough for my hair that it helps that a bit, gives it a lil extra shine, is really fine- not heavy and thick like most, and I like the smell :)
I know erin swears by a few of these things- I've personally shopped-down lip balm by
Kiehl's with her, watched her grip onto
dandelion (powder from benefit), and heard her swear by mascara by dior(?)- so I guess we all have our vices.