Why not? There's plenty of price data for high volume bandwidth applications, and even if there weren't, the parties could negotiate for it.

An end-user ISP like Comcast has a monopoly on access to their customers, so if they're allowed to charge for peering there is no competition to limit how much they charge. This tends to result in the ISP allocating 99% of the surplus created by peering to themselves.

That's very true. The only limit would be losing customer (HA!) because they didn't connect to the right content providers. There's some interesting network effects here that smell like positive externalities, which would explain the underinvestment. But also suggest that there isn't a good solution.