Maximum 750 for singles and 250 for doubles. It all depends on the opponent's ranking, if it's higher that win is worth more points. This rule does not apply to doubles if I uderstood correct. It's a bit complicated scoring system IMO.
It does seem complicated and I feel I want to complain about the ATP website and the site for the ATP cup that they don't explain it, at all. Also, on the ATP site, although they have the schedules posted for the first several days, as they are known, they don't list the actual rankings of the players, which seems rather an oversight, since they know this, and the ranking of the player has a bearing on the points gained, per win. So far, if you ask me, they haven't made it fan-friendly, in terms of what goes on.