A free one every so often even a reduction in price from time to time or a break when you move multiple toons at once. The part that they really hit a jackpot on is when an entire guild transfers, they get paid for each toon and the guild itself if people want to roll that stuff over. So every time a 25 person guild would move Blizzard would make about $1000, no wonder they aren't rushing to merge things.
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We can form up a few raids and do a speech in game over some body of water. Filled with bastard toons left for dead because at some point people don't want to keep paying to shuffle them around.