I buy mine at Ulta when they run a 20% off coupon. I think the prices are pretty even across the board. I also get a lot of deals buying directly from Urban Decays website. Sign up for emails for both sites and see which one runs a special first.
Sadly there isn't. Most of them just have a difference if a dollar or so. And if you ever tend to find a cheaper one it must be a replica. Please be aware as the replicas have some serious toxic chemicals which would be very harmful for your skin.