At least recently, I've been able to see all cabins. When I hover over a higher cabin (C+ or F), it shows me the price (in $ only) to upgrade.

The $0.01/mile to "pay" for the upgrade has also been around for a while, although I'm not sure it's always been available. I want to say it was only available during the booking process, so maybe now it's available anytime.

But since there's no award chart for "real" mileage upgrades, I'm skeptical that this feature really means anything new. Presumably mileage upgrade awards didn't give much more than $0.01/mile of value anyway...