Infrastructure costs money, absolutely. But infrastructure is largely a fixed cost (plus some maintenance cost). After that, it's all peering arrangements. ISPs here in the US can have some pretty ridiculous profit margins (probably exaggerated, but http://www.huffingtonpost.com/bruce-kushnick/time-warner-cab...).

With gigabit internet slowly becoming standard, ISPs do have to invest quite a bit in upgrading their infrastructure. However, that investment is easily amortized over time.

All that being said, the amount of data is never in play. Only the rate at which you can push data through that infrastructure matters.

Yes, data quotas help ISPs have higher contention ratios, but that's simply cost cutting and it hurts the consumer. Effectively, the same money you're paying now is buying you less.

Last-mile ISPs have a very nasty way of trying to lie to consumers about how the internet works. I highly recommend you read the Level3 blog posts from around the time of the net neutrality discussions. They're very insightful into how the internet actually works. Last-mile ISPs can be and are effectively bullies because they own that last mile.