Eyeshadow primer babble time!
On the left: ELF’s primer (the most basic one that you get for $1, in original since when I bought it they didn’t have color options)
On the right: Urban Decay’s Primer Potion in Original, retails for $20
Other bloggers I follow have basically said it before: the ELF one is good enough for pretty much all of your needs.
I really like UDPP, but some people have found that Too Faced’s Shadow Insurance works better for them. Personally, I have never had a problem with creasing while using UDPP. I have done all sorts of crazy eyeshadow things with it underneath and it has never budged.
The ELF primer works nearly as well- it’s not quite as thick as the UDPP and is probably just slightly less effective at preventing creasing, but the difference is so marginal as to be basically nonexistent.
If you have extra money or you can pick up UDPP under retail, then go for it if you prefer it (I do!). However, for the $19 price difference, there’s really not much of a reason to splurge.