If it was an over the water flight - it most likely was using using a satellite connection.

Those are always expensive and Singapore Airlines is passing the costs onward.

For example an Iridium phone is about $.75 for 9600 baud per minute, and Inmarsat BGAN is $450 for 100 MB.

There are other satellite services, but in the general aviation field that I know, Iridium and BGAN are the two biggies.