score:6
This will depend of course on how much you travel every day, but if you commute twice a day, your cheapest option will be to buy a rechargeable card (EZ-Link) and just pay the fare. Single fares while using this card are capped somewhere around 2.70 S$ a trip. The fare is distance-based, and transferring from bus to train or from one bus to another bus will still count as one trip. So assuming you travel twice a day, the maximum you spend in a week is going to be not more than 30 S$, very likely even much less.
One tool I like to use to find the fares for trips is the website gothere.sg you can enter your start and destination (if you can't find it, just use the postal code) and it will tell you travel time and price. There's other websites with similar functionality, but I find that this one has the most accurate travel times.