There are already many CA's, so the real answer is 'loads, but less if we think you'll just ask a competitor'.

But that wasn't the question. The question was "How much money do you want from us before we destroy your business?"

For one, there are so many CAs that could potentially cross-sign, it's unlikely none will "defect" and take the opportunity to earn some money, but also, if none had cross-signed, they simply would have started later with their own root-cert, destroying the business anyway, so that's another reason to earn some money while you can.