It's easy to remember. The SI prefixes are always decimal, that's why people call it "the decimal system". Kilopascals, megajoules, centimetres, always powers of ten. So a megabuck is 10^6 bucks, 1 million bucks, and a gigabyte is 10^9 bytes, 1 billion bytes.
The weird power-of-two prefixes get the special weird symbol and name, because you wouldn't want those anywhere except computing.