With BA search, since it always tells you how many seats are available, you dont need to put in more than 1 traveler in your search. This way you get an easier read on how much it costs PER traveler than trying to do the math if you are not good at it nor understand it.
The taxes and fee are shown when you select the flight. And you can go to the booking page then click the ? to get the detailed breakdown of the taxes and fees.
SQ has a very reasonable change / cancellation fee, but SQ charges YQ on award tickets. The YQ is the same as the pay ticket so you could find out how much by doing an IATA search.
BA as guv1976 points out, if you change / cancel ONLINE, you pay the less of the 2 between the cash you paid before and the $40 fee.
Always do the math to see if a cancellation actually works out better than a change when it comes to "lose" the previously paid cash portion. If the previously paid cash portion is a low number, you would be better off to outright cancel and rebook.