If you're travelling as a small group, you could charter a van. Those range from PHP1500-2500 or more. These are pretty informal arrangements. I'll try to look for a couple of companies, if this is the option you want.
You could probably get the same deal by dragooning a taxi. If you want to take a taxi, it would be a challenge to find one that would actually agree to go to Clark, and then theres the haggling, because its even more difficult to find a taxi that's willing to use a meter for a journey that long. Taxis should be safe, but its SOP for you to send an SMS containing the taxi's plate number and the driver's name as it appears on his ID to someone you know.
Or, you could just take a Victory Liner bus from Cubao to Clark. From the Cubao bus terminal, it'll cost you around PHP100 or less to Dau, near Clark. Buses leave every hour. The only disadvantage here is that from Dau, you'll have to take a tricycle to the airport (I'm not so sure if they have proper taxis in the area yet). Not advisable if you're carrying a lot of luggage.)
Well I have now used them twice for trips from the Angeles/Dau/Magalang area to NAIA and I have no complaints. The drivers have been friendly and competent drivers, and have arrived on time (in fact, about 15 minutes early in both cases, but they were happy to wait). The cars have been perfectly adequate, although being small-ish Toyotas they would be a tight fit for a group containing more than one person over, say, 5'9". The price is as advertised: PHP2400 inc tolls from the Angeles or somewhere adjacent to the expressway, a little more from further out (e.g. PHP 2800 from Magalang).