Obscure payment models are the future. At least until some governments realize that a market economy works best when there is transparency. This post is about Blizzard's offer to let you play Diablo III for free if you subscribe to WoW for one year.
It seems that any subscription you have at the start of the 12-months commitment will automatically recur as often as is needed to keep you subscribed for at least 12 months. You can probably change your subscription plan to 6-months and it would cost you a lot of money if you didn't. But don't trust Blizzard to tell you this upfront.
Anyway, this is just my guess and if anybody was able to figure this out with certainty, please leave a comment!
At the current cost of a six months-plan, the one-year plan costs 131.88€ and thus each month costs 10.99€. By taking the offer you save 58€ that Diablo III's standard edition currently costs at Amazon. These 58€ are equivalent to 5.28 months of WoW subscription time. (You can make the same calculation for one-month plans where each month costs you 12.99€).
Let us be clear that this is a bet. Blizzard bets that you would only be subscribed for 6 months or less during the next year. You bet against this! If, even without this offer, you were subscribed for just seven months of WoW playtime during the next year and bought Diablo III separately, this special offer would cost Blizzard 7x10.99€ + 58€ - 131.88€ = 3.08€ and they would never offer it to you.
Now, most players will lose this bet, because Blizzard got all the data and knows best about the future evolution of WoW. They know what patches they have planned, have done extensive market research and probably have a few informants at EA and NCSoft. And based on that information they consider it unlikely that the average WoW player, who agrees to this offer, would have been subscribed for 7 months or more during the coming 12 months.
Those players, who do not agree to this offer, are probably the majority. And it is probably a safe prediction that these players are less enthausiastic about WoW. And as such, on average, they will be subscribed less months of the coming year, than the more enthausiastic players who accept the offer! Thus we can clearly see that, in the absence of this special offer, Blizzard expected the average player to be subscribed for considerably less than 7 months out of the next 12 months. I wish I could write this down in a way that it is easier to understand, but Blizzard did their best to make that really difficult.
Anyway, just keep these thoughts in mind before you decide that you are the exception and going to win that bet!
I ignore the fact that some players have considerable time left on their accounts, because they chose a 3-months or 6-months plan last time they resubscribed. This fact has some influence on the above calculations, but only Blizzard knows the numbers here.
Keep one thing in mind: By using a 6-month plan you pay 10.99€ instead of 12.99€ the month. That means that you save 2€ a month and 12€ during the six month period. One day of a 6-month plan costs you about 65.94€ / (365/2)= 0.36€.
Usually people do not resubscribe immediately after a subscription has run out. Sometimes they are on vacation or it's a weekend. Sometimes it's night and you don't immediately jump out of bed to resubscribe the second the subscription runs out. If, using a 1-month plan, you end up not being subscribed for about 12€ / 0.36€ = 34 days, the 1-month plan is superior to the 6-month plan.
Now, that seems like a lot. But keep in mind that Blizzard wouldn't offer you to save 2€ a month if they thought that this loses them money. Blizzard has done some market research here, too. And apparently they found out that the average subscriber of a six-month plan, would be subscribed for 34 days (or more) less if he had used a one-month plan.
You can generally assume that any special offer turns out to be profitable for the company, and is thus a bad decision for the average accepting consumer (not the average consumer!). But you are not average, of course :)