UDPP: $17 for .34 oz
Both the ELF primer and UDPP are the same color and apply totally sheer. ELF's MP is almost watery while UDPP is thicker but performance is about the same. I've had days where ELF's MP creased before UDPP, and days where UDPP creased before ELF's MP. For both products they usually don't crease until 9-10 hours after application.
Due to the price difference, and the fact that eventually you have to cut open Urban Decay's Primer Potion to get the rest of the product out, I'd buy the ELF primer over UDPP any day.
Check out my post on L'Oreal De Crease vs UDPP.