Now I realise...
On the first night, I took a bus from Tsim Sha Tsui to Mong Kok
and ended up paying quite a lot of money (ard HK$8-9+), prob
because I chose a long distance route which just started. I didnt
realise the cheapest route was only HK$4.40!
A couple of days later after taking a closer look at the various
bus svcs and bus fares, I realised I got to compare the bus fares
first to avoid overpaying (in cases where you are travelling short
distance and there a few routes to choose). No wonder majority of
people avoided certain routes for short trips and the one with the
cheapest fare was the most crowded! 
For the XH/expressway routes you mentioned, how do the fare
system work since they are not fixed fares?
Ah TST to Mongkok, passengers will onli take 1,1A, 2 or 6,9 and
not other rts...
Eg. pple in Admiralty goin to Causeway Bay will take 5,5X rather
than XHT 962...
For XHT, eg rt.112 before the Harbour, it is HKD9.30, after harbour
at Causeway Bay it's HKD5.70 all the way to North Point