Disney has shown that they are willing to price each park in tiers, they can do the same for an express pass-style system.
Assuming a very slow 20K attendance day at MK:
Taking the average Genie+ price at $18.50, and 50% of park guests purchasing it, that's $185K in revenue.
If you took 3.5% of all park guests (700 guests) and priced an express pass system at $300, that's $210K a day.
Willing to bet there are at least 700 guests willing to purchase at that price.