The problem with the suggestion of ambrust is obviously the threshold effect caused by the 8 to 9 players "wall". Therefore, I suggest we integrate the methods of tax systems to avoid the threshold effect. That is, you first count the population of the server. Then you split this count between the different "taxation rates" you have, using the population thresholds associated to them (here, we have the "0-8" zone and the "9->" zone). Then, you divide the upkeep between the zones, weighted by population. An example :
The server has 24 players.
There are three zones :
1) 0-15
2) 16-32
3) 32+
The 1st zone represents 16/24 of the global server population and the 2nd zone represents 8/24. The third zone is outside the current server population.
Let's say your upkeep this round was 1000 gold. You'll have 1000*16/24 gold applied at the first rate, and 1000*8/24 gold at the second rate. So if the first zone had a reduction of 50% of upkeep and the second zone only 20%, you would pay 1000*16/24*50% + 1000*8/24*80% net upkeep.
If we apply this to the 0% upkeep under 8 players suggestion, and full upkeep over that, we get that you'll never pay for the part of your upkeep equivalent to 8 players among the total server population. Which means that you'll pay nothing under 8, 1/9 upkeep at 9 players, 2/10 upkeep at 10 players, 3/11 upkeep at 11 players etc. EU_1 usually has 100 players, which means they would pay 92% of their upkeep.
To make it really fair and abuse-proof (not completely but atleast as much as the current system is), the minimum upkeep rate shouldn't be 0%. Furthermore, having only two different rates seems not adequate. 8 seems like a good spot for the first delimiter, but it seems to me 24 players should also mean something. Let's say 40% upkeep under 8 players and 70% upkeep between 8 and 24 players.