Is the quota simply 10 X the number of entrants? If so your formula would look something like this:
3 + (buyins * 10) * switch(rank, 1, .45, 2, .25, 3, .15, 4, .1, 5, .05)
This would give each player 3 points, and 1st through 5th place would get 45%, 25%, 15%, 10%, and 5%, respectively, of the points quota. In a tournament of 30 players, 1st through 5th places would get 135 points, 75 points, 45 points, 30 points, and 15 points, respectively.