When you install the Verizon software - there is a page that tells you officially how much data you are using - according to Verizon and how they will bill you. I have been using Verizon broadband for travel since 2006 - not as my primary. Even with heavy travel - i.e. constant use for 2.5 weeks this last Februrary - I stayed below 5GB. (Maybe 4GB so it was close)
I am a heavy user as I have a lot of software and linux distros that I need to download. I don't think 5GB or 10GB would be enough for me, but if all you do is web, email, facebook and games you could probably easily stay below the 5GB.
The best way - is as above - monitor usage with a program to get a feel of what each page uses.
But to answer your question when you hit a page you send a http request (which is data) then the server sends you the web page (which is also data) some pages have autorefresh(like facebook), games can be constantly sending and receiving data as well - not always - i.e. flash will usually download the game as a program and it runs locally while you are on the page. As stated - there could be ads or whatnot on the page which could be sending and receiving data. Independent of the Web your computer could be sending and receiving data like network time updates, software update checks, bonjour checks, etc. Another good program is called little snitch - helps you monitor what processes are trying to communicate on your computer.
Little Snitch
After installing - I found there was a surprisingly large amount of software that wants to send/receive data.