The option on the subcontractor contract drives the cost. You register the revenue from the client as a separate Change Order Client. Keep the options on the contract, and use the Group field to collect the client change orders that concern options.
How to do it
Go to Purchasing, open the contract and exercise the option. Select Change Order, and a Change Order Subcontractor is created with the cost.
Go to Change Order and carry the Change Order Subcontractor forward to a Change Order Client, or create a new Change Order Client.
Enter the revenue from the client, with markup.
Set Group to, for example, "Options".
When several subcontractors belong to the same option
If the option towards the client concerns work from several subcontractors, you exercise the options on each contract and link them to the same Change Order Client.

