After Teenage Dream, I'll be really disappointed if Prism doesn't debut with at least 800K. :\
ARTPOP could sell 500-600K on its first week, but I'm here to a longevity era like TFM.
Probably Katy will have lower sales for first week, but I think her album will have more longevity on charts than Gaga's , just like happened with "Teenage dream" and "Born this way"
I'm sure Gaga will debut with more, but Prism will probably have more longevity and sell more in the end and generally be a much bigger era, much like TD/BTW