Not to give Disney any ideas (although I don't think that they'll implement an express pass option that lets you ride any ride any time you want - they've got too much invested in My Magic to upend or end run that $1.5B system), but I'd expect something more along the lines of $400 to $500 per person per day for one park. As
@dlavender points out, Disney already is charging about $80 per person to ride 3 rides for about an hour at MK (EMM), another $80 to ride FEA once (yeah, there's a dessert party and fireworks, but FEA is the draw), or a whopping $250 to ride FoP once (yes, there's RoL, five other rides and a sampler platter - mostly window dressing to give you additional "value"). There'd be a ton of demand for something like this. Price it too low, and everyone has one (negating its purpose). But as I said, I don't think we'll see this anytime soon. What I do think we'll see are more "experiences" that are upsold and combine food, rides, fireworks, etc.