We didn't test this with Express Pool but Pool in general is a different animal since the prices are so cheap to begin with, there's not a lot of variability that can come into play and affect the pricing.
Plus, Uber takes a huge loss on Pool and Express Pool especially when they first launch since they want to offer low prices to riders but they don't know the match % rates yet.
Can you qualify what you mean by a frequent rider? How frequently are we talking about?
I did experience lower prices when I first started using express pool (though not to commute to/from work), and at one time I got a $2.50 ride to go from downtown SF to Ocean beach (a 40-ish minute ride), which anyone with half a brain would suspect to be VC subsidies at play.
Could it be that this is a similar case, where the original "cheap" price is just subsidies for new rider/some desirable target audience and the "ripoff" price is just the regular price if VC money/competitive race to the bottom weren't in the equation?
Another theory I had was that congestion in pick up/drop-off areas would play a role in pricing (given that cheaper-than-expected rides happened to be nearer the freeway as opposed to, say, Market st)
Plus, Uber takes a huge loss on Pool and Express Pool especially when they first launch since they want to offer low prices to riders but they don't know the match % rates yet.