MC/BWL are very heavy on bandwidth....so yes you will expect a lot more traffic during MC/BWL Raids...in most 40 man however not on the 5/10 and 20mans
Let's not forget if you use Team Speak or Ventrilo Voice programs during these raids traffic will be pretty heavy. A good raid can wipe you out 100-200mb...Ie BWL + Ventrilo. From Start to Finish.
A "budget" solution may be Nukecap's unmetered VPN if your on an IS ADSL Account...Since it uses local traffic for VPN then connects to Int servers...letting you have 30gig of traffic for WoW (Euro or USA). However .... I am not sure if Clipse will change the rules for Unmetered since a lot more people doing MC/BWL now...and Unmetered i dont think can handle 5kb/s-10kb/s per a raid for 4-7 hours weekly. Best ask him. This is in "theory" a great and by far an affordable solution for what you need.....Especially if your voice server is based in SA. This comes at a different price though....IS network is very unreliable....often peering issues every week (as you can read on the general ADSL Topic) Causing the VPN pretty much useless also not worth using the VPN on an unshaped account as unshaped generally has a better latency it seems. To euro that is (100-200ms diff) This is the only solution for WoW on IS account I can think of...Otherwise for gaming i think IS is the pits! Avoid it if you can
If you got cash to spend....you could go for an Imaginet Freestyle+ account. These are unshaped uncapped accounts. Basically you buy an unshaped package...Let's say at 4 gig for R460 p/m...Any usage over that will be on your next bill at R12.50 or so per 100mb (Dont quote me on prices though)
Another solution i found most WoW players use is Webafrica Pre-Paid Unshaped accounts....I believe it is R125 per a gig of unshaped bandwidth...Good side being is you dont lose your bandwidth end month..your left over traffic rolls over to the new month...making it a great way to top up on the go (which is instant) bandwidth on demand
Personally i would go for the SAIX accounts above. Just for reliability and latency...On avg with Unmetered VPN i had 500-700ms.....Unshaped accounts i have 300-550ms with the rare 600. (fantastic for raids/pvp) It's also nice seeing USA people on Euro WoW having a worse latency than you
As for the addons...Yes and No....It all depends on the addons you are using. If you are using an addon that uploads and downloads information for location spawns, item sync, population ratios etc then yes expect to use bandwidth. General addons that only change UI/Hotbar/Macro's wont use bandwidth