I think the cost of WoW has been fairly reasonable over the last 20 years. The subscription has stayed the same price of $15 per month the whole time, and if you know you are going to play WoW for the whole year, it is just $13 per month with an annual sub. The only thing that has gone up in price is the expansions which have gone from $40 to $50.
I think another point that you are missing is that WoW players often spend most of their game time playing WoW, which means they spend less on buying other games.
Battle.net games run well on Linux in general. The only issues I usually have is with their client itself.