I read many people enquiring on the transport from Clark airport to Manila, I just came back from Manila so I could share my experience here.
There is an air-cond shuttle from Clark airport to Manila costs about 350 pesos. However, a cheaper alternative is to take a taxi to the nearest bus terminal( cost 100 to150 peso), then an air-cond bus to Ermita's Quezon Boulevard will only cost about 135 peso or 75 peso depending on which bus company. They are all non-stop air-cond bus, pretty convenient, about 45 minutes to one hour to Manila.
However, if you think of doing the same from Manila tp Clark Airport, you must allow plenty of time like 4 or 5 hours in advance. We nearly missed our flight due to a jeepney driver. The bus dropped us to Shoemart (SM) which is pretty close to airport, we couldn't get a cab, so we hop on to a jeepney after a few reassuring yes to our question of Clark airport. Instead of the expected 10 minutes journey, he took 45 minutes to drop us to a far away taxi terminal, then cost us another 200 peso taxi fare to go Clark airport.
The hassle free way from Manila to Clark airport is to pay 2500 to 3000 peso for a cab direct to the airport terminal from Hotel in Manila.
If you need to take a taxi in Manila, watch for those coloured one with Company name that will honestly use meter. Try NOT to use those white taxis, those are mostly personal's. They always extort 200 to 300 peso for a short distance that may cost you a quarter of it by meter. We had too much bad experiences as such, they promised to use meter when we hop on, then ended up going round the world till 250 peso instead of paying 50 to 60 peso by meter.