It's only the NPC's position and action at the time that loads through the internet.
Yes, I meant to say that. The server sends the client what object to load where and if it moves like a NPC it sends movement data for each NPC in packets, so this increases the amount of data transferred over the network the more stuff come into picture and move around, before they disappear.
This is not enough data to do an estimation, so your estimation for "worst case" (highest bandwidh used) is very low. Danellos' number, over 60MB/h is much more accurate estimation for raiding. There are many factors though, how fast you pull, how long you raid, how much you idle, etc, so your numbers may be different. I'd say 3 25-men raid nights will use up more than the half of the OP's 1 GB plan.
When I measured WoW traffic it was usually between 180-200 MB for 3+ hours of 25-men raiding.
I agree it's not something universal, just from that one experience that I mentioned the data was taken from. Still it indicates more than just assuming and guessing, like I first did before logging anything I guessed something way off what I found. :D
In short what I can say, chances are you will spend your 1GB in half month or less, so it's not optimal. Also don't run addons with background traffic
, if they sync all the time you use more bandwidth to send and receive addon data. So consider this as well.
Ran a LFR, no wipes, all first time kills, including trash. Average bytes per second was 9344 and it took me nearly exactly 1 hour to do the whole first part of Dragon Soul, hence 1 hour ~= 32MB of bandwidth.
I didn't use background syncing addons of any kind to reduce the load on purpose, no heavy duty communication going on, only the game default routines.
You can debate the accuracy but it will vary, in my situation it used 32MB in LFR for 1 hour (full first part only), I idled Stormwind shortly after (~200 people in the city) at the hotspot where most are, the game used about 5120 byte per second and I idled about an hour so 1 hour ~= 17.5MB of bandwidth used for idling in one of the biggest places with most people.
You can do the math yourself, I think this is one of the best scenarios that can happen, if you like to be safe double the values so you consider wipes and waiting times for replacements, roughly you will at average spend 96MB of bandwidth per raid, plus 2 hours of dailies and idling can take around 35MB more so you'd spend 131MB per day at average.
In guild you don't LFR so you do more tries, more wipes, more waiting, quickly one raid stretches across several days, I remember I raided 4 hours and 3 days a week roughly to finish one new raid (progress raids), so if you extrapolate you can estimate about 128MB per day just for raiding, quickly turns into 384MB per week, before you know it you will spend 1GB in the first two weeks and probably earlier, and this is not even including dailies, random idling, having fun, battlegrounds, e.g. :P
Disagree if you like, I just told you how I think things would go down based on what I experienced in the LFR. :P