Most game data files are already compressed.
You'll only get a 10:1 or 5:1 compression ratio on sparse files. i.e. Things like spreadsheets filled with lots of zeroes, or small Word files which have a ton of empty formatting info.
For more typical data, a realistic compression ratio to expect is about 80% the original file size (5:4). Depending on the content you may get as good as 70% or as little as 90%. But around 80% is a good average to expect.
Text files will compress more because they only use about 70-100 possible characters per byte, while a byte is capable of representing 256 different characters. So you can expect to compress them to about 50% to 25% their original file size (2:1 to 4:1)
Compressing an...