[Editing for everyone else rolling by: I just checked, middle was cheapest, window was mid price, aisle cost the most. Original comment continues:]
90% sure the middle seats are cheaper when I book United directly. I fly 6 trips a year and am a window addict. I don't know, doesn't matter how often I fly, I stare out the window. It's fascinating every single time. I also don't have to get up much, so I enjoy not being bothered for everyone else's bathroom trip. My SO will pick the middle seat, which is why I'm pretty sure I've seen it be cheaper.
I have also been burned with a wall seat. I got it on some Embraer (175?) where it was 2 seats each side, I think row 10 on one side and 12 on the other were walls.
I will acknowledge the aisle seat may be better on this new-to-me annual 15 hour flight where the 787 windows were tinted the whole time anyway. Sub-8 hour flights are fine, but the slow creeping of claustrophobia is real on that flight.