"It's all going to be taxpayer's money no matter which budget it comes from"
I think John Brown's point is valid. If it's heaped on ISPs it's hidden, HMG doesn't have to justify the amount & they'll claim the fact it was 8x over estimate as the ISPs' incompetence. If it's a matter of public record they'll have to justify it and it becomes much harder for them to hide the real cost. Even better, if they stick to their estimate of cost and then have to fund it directly the Treasury has the chance to turn down extra funds whist the PAC & NAO have a chance to give them a good kicking.