So, there should be a calculated amount of points that is the buy-in times the initial number of tables, and then 1st place should receive 100% of those, 2nd place should receive 80% of those, etc. Right?
If so, here's a formula:
(buyinCost * ceil(n / 10)) * switch(r, 1, 1, 2, .8, 3, .6, 4, .5, 5, .4, 6, .3, 7, .2, 8, .1)
The only problem is the formula cannot know how many tables the tournament started out with. In version 2.6 (currently in beta testing), the variable tablesLeft is available, but the value of this variable will change as tables are consolidated and at the end of the tournament when the final points are calculated (points are continuously recalculated throughout the tournament) this variable will likely be "1" and thus won't be valid for your formula.
So you have to code into the formula how many seats are at each table (and of course it assumes each table has the same number of seats). So, this part of the formula:
ceil(n / 10)
computes the number of tables needed at the start of the tournament. If your tables have, say, 8 seats each, change the 10 in the formula to 8.