there would have to be a cost - however the mitigation is AirTankers costs are likely to be minimal if the decision were to be made now, and thereforefairly easily absorbed by MOD - and political: if AirTanker quote a ridiculous price the MOD can start making noises in the media about dodgy PFI deals, and whether its lawyers should start looking at whether they can be ditched.
the government has a somewhat larger stick than AT - it writes the law.