This is a common claim that is rarely supported with data. Why does enforcing net neutrality increase downstream costs?
If you mean that it shifts costs (you pay more for Netflix because they have to pay Comcast for fair service), then sure. I don't see any evidence that net neutrality actually changes the underlying cost of net service, though.
There's a great counterpoint to your claim in the USPS. Their prices are regulated by congress. Those prices are pushed down artificially to the point that it's slowly bankrupting the USPS. So service is definitely cheaper than it would be without regulation. (Send a letter via FedEx and see what the free market charges for this service.)
Bandwidth usage whales are just those that constantly use the bandwidth they paid for - which, outside of those peak traffic windows, isn't actually costing you anything to provide, because you aren't saturating your bandwidth anyway.
If ISPs want to cut costs, they rate limit customers (globally or during their peak usage times). That reduces the amount of routing hardware they need and the amount of backbone bandwidth they need to buy. If they institute data caps or try to break the Internets P2P architecture, they are just destroying the intent of the Internet while still having the same peak bandwidth rates they always did. Their routers just run cold now at night because nobody is using them for anything.
Can you go into a little more detail about "given" here? I'm not under the impression that it's true.
Again, this is the typical scenario; there are certainly cases that don't fit this mold, but they would be atypical.