Mistershine on 07 March 2008
Games are fixed at 1000 points on retail games, 250 can be added with DLC though, and XBLA games are 200. How the points are given is down to the dev, so it varies as to how difficult they are. As you have already mentiond Avatar is stupidly easy to get a max score, whereas some games are really hard to finish. IE: GRAW required you to climb to the top of the world leaderboard for an achievement and Gears of War's 'Seriously' needs 10000 kills in versus ranked matches.
