Someone did the math on Hearthstone about a year ago and when you throw everyone together the average player spend roughly $0.30/month (perhaps slightly higher, since some people might have multiple accounts). That's $3.60 a year - the cost of a single paper booster pack.
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Yeah, the number is based on Hearthstone's reported revenues and their reported number of players (both of which are public). How much of the money ends up in Blizzard's pocket is anyone's guess since we don't know their costs.
For the record this metric is called ARPU (average revenue per user) or LTV (lifetime value). It is largely used to determine the quality of users you are getting from user acquisition in free products with IAP.