Expenses are relative.

The initial cost of the game might be $40 (Classic + BC Battlechest) +$40 (WotLK) +$40 (Cataclysm) = $120 per account.
I'd imagine a new battlechest will be out eventually, everything except that latest expansion.

Boxing costs about $40 for each new expansion per account.
If you are five boxing, thats $200 every other year.

Then you average $15/month per account.
Boxing software ranges from Free to $20 for a lifetime license to $36 per year.



If I were to average my five boxing.
I spend $8/mo on the current expansion, $4/mo on my software, $75/mo on subscriptions.
Under $90/mo, which (to borrow from others relative cost comparisons) is less than my cell bill or one date.

A pretty cheap hobby, overall.