Consider, your ISP knows most of this information. For instance, if you have 2 GB of 4G cell service and then it throttles to 3G, then the cell company must have real time info of how much data you have left, or they would not be able to throttle you. They will often give you an app so you can check how close you are to the limit. The only trouble is that there is no STANDARD way to ask them any of this.
So make one up. This is one of the few cases where network effects mostly take care of themselves. Here is a straw man proposal. Feel free to bike shed this. Let's say we extend DCHP so that in addition to giving you the default gateway, it also gives you cost parameters for the current connection, or possibly the IP address of who to ask for the cost.
If the network supports this, the OS can find out when connecting and then provide it to apps via an API. If the network doesn't support it, the OS can still expose the API, but fall back to heuristics such as whether you're on cell or Wi-Fi. Of course, the user typically knows what they are paying, so you should allow them to override the heuristics and supply real data. The apps need not know whether the data came from the network of from heuristics or the user.
The phone itself could easily record the returned file size of all requests within a threshold. My old Note3 has these alerts built in with separate limits for mobile vs wifi.
Deploying network infrastructure is the hardest and most expensive element of networking. You want to do it as little as humanly possible, and anything that can be retrofitted on top of already-deployed hardware is an instant winner.
Changing network protocols would likely require hardware replacement all over the place.