I can see it now. Adrian Newey starts his own company and signs a contractor agreement for $1/year to provide technical assistance to Red Bull Racing. Red Bull GmbH features Newey in a commercial and pays him $100M in appearance fees. Maybe Renault provides engines to Red Bull Racing for $1. Completely unrelated, Red Bulls pays for advertising in all Renault user manuals to the tune of $50M/year. Red Bull Racing has only spent $2 so far.
Which is why the FIA needs to employ professional accountants to police the cost cap. The money spent on each component and its origin will be detailed somewhere, a proper look through the accounts will make it extremely difficult to hide the payments. Even if you do start playing silly buggers through subsidiaries and the like. The teams have an advantage over the FIA on the technical front, by having the resources to hire large teams of the best engineers in the business, which has made it difficult to write the rules as recent controversies have shown. But for this, providing the FIA does it properly, the teams would need to burn through large piles of their own money hiring outside experts to try and find ways of hiding extra spending.
I would imagine the rules would have to be structured something like this:
1. You get $X million in total. Possibly this will exclude driver salaries, marketing and other sundry expenses.
2. Full accounts containing total spend on each component, engineer salaries, payments to external suppliers, etc, are to be made available to the FIA.
3. As a condition of entry, accountants employed by the FIA have the right to go through the teams books to ensure compliance.
4. Any violation of the rules can be penalised by anything from a reprimand to exclusion from the championship.