When do you use bytes, only when downloading or when browsing in general?
You are ALWAYS downloading something, being it a HTML page (a site where you read stuff, forums, blogs), images, videos, or just program-specific data traffic like games and skype or whatever. That's data that moves from a server somewhere in the world to your device through the ISP's connection, what it is is totally irrelevant, everything is counted towards that MB-per-month goal.
Disclaimer: all numbers below are estimates, use at your own risk.
I play no online games
0 MB from this, also good for your interaction skills
I browse text-based sites and blogs, forums like this one
A full page of a blog or discussion forum like this is in the hundred of kb range, (downloaded a page from Peak Iron thread in Science section and it's about 200 kb, you can do the same for your favourite places to eyeball on your own) but especially cool-oriented forums with fancy interface and fucktons of ads may force you to download much more.
Use Adblock plus to nuke ads (available for both firefox and chrome). You don't want to see them and they gobble significant MB if you have it capped.
sometimes look at pictures
15 medium-quality porn pictures (big as the screen, but not the 50 megapixel overkill kind where you can zoom in and see every single pubic hair) weight around 10-20 MB.
, and I enter YouTube pretty much every day.
This is the real killer. Videos are SIGNIFICANT weight, especially high-quality ones.
a video like this
(7 minutes, done and uploaded by professionals, it's not homemade crap)
if downloaded with firefox's downloadhelper (downloads whatever goes to the browser's player, so it's the size of the video you're watching, not the overall page size, albeit it isn't that big compared to the video anyway) weights:
16.4 MB for the crappy 240 p quality
34.7 MB for the acceptable 360p qulity
51 MB for the good 480p quality
103 MB for the overkill 720p quality
170 MB for the over-the-top 1080p quality
Also, AdBlock plus nukes horribly annoying Youtube video ads as well (that gobble up significant MB for no good reason).
Theoretically streaming a movie through a site that offers them for cheap means around 800 MB - 1 GB per movie if they know what they are doing, if they are idiots it can be much more.
I now have unlimited MB per month but the price is a little steep in phone calls.
I'd say keep this and use skype for calls if you can.
Having unlimited MB is kinda critical if that's a contract for a home, and becoming pretty damn rare for mobile devices.