For those interested a "Metered Taxi" fare from Silom Road & the Tollway intersection to outside of Khao Yai's main Gate is about 1,700 Baht plus two tolls of 50B & 10B. (180kms) Total time was 2 hours 20 minutes with a very safe driver starting at 9 AM. After which we stopped and paid for his lunch. I did this as a whim to too see how much of a hassle it was. It wasn't. Several other taxi drivers wanted 2200 to 3000 Baht with no meter. Obviously the fare will vary from your starting location but now you have an idea of cost if you elect to take a metered taxi.

3000 baht obviously taking the piss, that is 5 hours of travel time back and forth....

That's a interesting piece of information. When you got to the main gate what did you do? Take the same taxi back to Bangkok or hang around the park?