Cheapest is to walk, though god knows when you get there...
Generally, it saves the environment mainly due to the energy
consumed per person per km travelled. Of course a bus consumes more
energy than a car (with it being a heavier vehicle), but consider
the number of cars not used when the car users switch to bus.
I doubt anyone really cares much for the environment (except for
the young green yuppies) since most people I know would very much
prefer a car. Think of the convenience to go anywhere and NOT
having to worry about the LAST bus/train! But in some instances
when parking and traffic jams are major headaches, having a public
transport is more convenient. No need to worry about where to park
your car or worry about accidents/scratches while being caught in
the jam.
As for from JKN to CGA, MRTs win hands-down. Generally, my
personal rule is any travel involving 4 stations' distance or
more will be faster on an MRT. (Different people have their own
rule of course)
hah? cheapest? i'll say the most expensive.
imagine how much water you need to buy to hydrate yourself
through the walking. 1x 1.5litres of water from 7-eleven cost about
1-2 dollars. And even if one gets the water from home, minimumly
the bottle still needs to be bought right? And if really want to
save money and get a bottle from a dustbin (yuck), how about
thinking about the food that you'll eat when you rest...(and yeah
it only applies to long distance walking)
And let's say if you don't do all these, i bet one can't reach
Changi Airport from Joo Koon safe and sound. You'll end up in an
ambulance and hospital bills will start bombing you.
In terms of environment friendly, I think that there's no 100%
friendly to it. There's bound to be some form of small damage to
the environment.