Blizzard store per month:
€12,99 for a 1-month sub
€11,99 per month for a 3-month sub
€10,99 per month for a 6-month sub
Online 3rd party: €20,98 for a 60-day card (€10,49 per month)
Edit: 4,5% to 19% cheaper
And that's just a Dutch online shop. I would be surprised if there aren't any cheaper out there.