right now WoW Battlechest's (vanilla wow + TBC) are going for around 30-40 dollar, depending on store and if its on sale, plain cd-key's online are a bit cheaper, but than again, no box or install cd's

you can send him an invite trough the wow site, so he can start on his 10 day trial (and automaticly linking his account to yours, meaning a free mount for you, and 3x xp bonus when you AND him are leveling together in the same party with no more than 5 levels difference between each other, so no boosting)

make sure he has wow installed before starting the 10 day trial.. wow site does offer a download version, but that wastes the first 2 day's of the trial on download time