Oh, how many times have I heard this ...
My understanding is that if no figure is stated in the Contract Data, then a "fair and reasonable" percentage would be imposed by the courts. (It is definitely not a zero figure) ! I.e. what would be a reasonable market rate for a that type of subcontract.
So far better for the Parties to agree this by discussion, than by resort to legal advisors.
Far better to have it in the contract in the first place.