Nexus 7 -- $200, but you'll have to side-load flash player on there (Flash Player officially doesn't support Android 4.1, but works flawlessly on there). Limited local storage.
Transformer Pad 300T -- little less than $400. It's basically the non-HD version of the 700T. Has the option for a snap-on keyboard dock that adds more juice to your battery life (additional $100). It officially supports Flash Player for now, but once 4.1 upgrade hits it (ASUS announced it'll get the upgrade), you'll have to side-load it.
I wouldn't bother with the slider or the original transformer prime. If you want to spend $500, go for the Transformer Pad 700T.