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.