With Netflix you can set the resolution for watching programs using the Internet. We found that we could use the lowest resolution setting and had no problems with the picture quality on our 52" TV. The upscaling from the DVD player is probably responsible for the great quality.
With a TV show that has 42 minutes of viewing time the data usage is about 275MB at this resolution. At the highest resolution setting the usage is closer to 3GB and you are not going to see a difference with most of what you watch.
We get free movie streaming with Amazon Prime but have not been able to determine how to reduce the resolution being provided so do not use it.
The Cradlepoint 95 and 1400 routers that provide Wifi as WAN can be used to track actual usage in real time. We do this with our Millenicom service so we know day to day where we stand with our 20GB cap. These routers can also be easily set to send an email alert to your computer when you reach a level that you set for advance warning.