Steam initally launched in 2003 as a updater/server browser for Valve games like counterstrike, half life, and team fortress classic. Apple music came out earlier that year, which isn't a 1:1 relation but likely influenced things wrt download pricing.

Steam didn't have its first third party game til late 2005 which puts the chance for it to standardize a rate for game downloads right around the timeframe of xbox live and psn launching (late 2005, mid 2006 respectively), so I wouldn't be shocked if word got around the industry about that stuff, though that's just me making reasonable logical deductions (People love opening their big fat mouths, lot of folks in the same circles, etc.) rather then anything solid.