How Much Broadband Does a Household Use

My wife and are planning an extended car trip out west, renting an RV. And although the places where we might pull in at night may have Wi-Fi, we were thinking about spending the $20, and purchasing one of the USB laptop modem aircards, for some connectivity while on the road. The aircard vendors we've looked at all have pretty good coverage in the areas we're going to be traveling. We're not huge broadband consumers, but I see that you can purchase prepaid blocks of time for set prices (300MB / 1-week / $15). Candidly, I don't have any clue how much our broadband usage is per day / per week - although I suspect it's either much more (or much less) than I think. Which really isn't helpful in narrowing this down.

Anybody have any ideas of what a household of two would use per day, where the occupants are not streaming tons of videos or other high capacity downloads/uploads (and no high-consuming youngsters)? Is there some way to query your Wi-Fi router for how much "traffic" is running through it?