$250 from Admiralty is a fair price. $170 is from Kowloon only. With the slightly longer distance, and the toll for the tunnel, I think $250 is just right.
Either way, it's about HK$100 off the metered rate.
BTW, the driver has to start the meter, or else that'd be too suspicious with the flag up and the light on the roof still on (if it's at night).
And, yes, basically all taxi drivers will offer to take you to HKG at a reduced rate if you tell them to go to a Airport Express stop, unless he's about to go off-duty. A HKG roundtrip is HK$600 of revenue - it'd take a whole day driving around the city to get that.
[This message has been edited by rkkwan (edited Mar 02, 2004).]