They've got several advantages over a traditional data recovery service. They don't need to buy obscure parts at retail, or buy drives and cannibalise them. Also, there's no need to reverse engineer a drive.
They can probably also offset some cost by using the large sample of failed drives to find frequent failure modes and designing them out of the next generation.
It's also $30-50 for any drive. If 1 in 10 fail and are asked to be recovered, they've got an effective budget of $300-500