I think the range is so widespread that the "average" would be hard to pinpoint. I know many composers get paid per minute of music delivered. An old teacher of mine said he got paid like $200 per minute of music that he wrote for a game, and that's considered "low." If you deliver 5 full minutes of music at $200/min, that's a decent chunk of change (at least EYE think so), unless you're doing a project of that size only once every 6 months. Big time composers for AAA titles could potentially make, e.g. $600 per minute of music delivered, and in a big game that requires over 100 minutes of music, that's a lotta money. Not to mention because they're already writing for a big title, they've proven at that point that they do good work, are reliable and responsible, so that reputation will garner similar projects of that caliber.
If a composer wrote music for one big game in 1985, they probably moved on to other stuff; I highly doubt they're still living off the money earned from it.