Never tried it and don't know anyone who has but you can try just e-mailing a legit coupon taxi company and see what they have to say about the prescribed rate. I think Nissan is about the biggest and reputable airport taxi operator at MNL.
http://www.nissanrentacar.com/services.do?id=942
[email protected]
Also, beware all manner of coupon taxi misdirection scams where clean cut guys in white shirts outside arrival try to direct you to "limo" taxis and beware any driver pulling out a laminated card to show you what "rates" are.