I think that as a factions size gets larger the per-person tribute should decrease (which would encourage larger factions) rather than increase. This is for the simple fact that once you get more than 40 people the tribute has increased from 180 (max tribute of first bracket) to 3,920 (max tribute of 5th bracket [40-49 people]). That is 21 times the amount for 9 people [note, the population has increased by 5.4 times), which is INSANE. A faction with 80 members (which isn't common but happens to be about the largest sustainable population due to the rate of inactivity v/s recruitment) must pay a whopping 8,000r to cover those people which is 44 times the max tribute of the first bracket. I think that it should go as follows:
Bracket 1(1-9): 60r per player (60 to 540) - | - (450 more than zero lul)
Bracket 2(10-19): 50r per player (500 to 800) - | - (1.5 times larger than B1)
Bracket 3(20-39): 40r per player (800 to 1,160) - | - (1.5 times larger than B2)
Bracket 4(40-49): 30r per player (1,200 to 1,470) - | - (1.8 times larger than B3)
Bracket 5(50-59): 25r per player (1,250 to 1,475 ) - | - (1.3 times larger than B4)
Bracket 6(60+): 20r per player (1,200 and up - | - 80 players = 1,600) - | - (1.1 times larger than B5 for 80 players)
You may be saying "But Mechish, that causes the max tribute to drop after entering the next bracket!" to which I reply: being a larger faction should not result in a punishment from the server. The server recently initiated a shrinking of the min distance between factions because there isn't enough space to store all these tiny 1-20 player factions. With my bracket system the general cost of tribute for a larger faction goes up, but much less exponentially than with the current system. It ensures that small factions still need to pay more than 20r after getting involved in stupid warfare, while also ensuring that larger factions have more risk (but not an insane amount of risk). This will lead to larger factions, stronger communities (within and between factions) and more massive cities/settlements. You may actually see factions get over 100 players and sustain that level of population, with actual "cities" (by minecraft standards) being built. It would declutter the worlds of small, often inactive, minifactions that don't have any reason to increase their population and every reason not to.
Every single one of my factions that got over 50 members collapsed because I got tired of be attacked for the tribute. The largest faction I had held over 100 players and had a tribute (under the current system) of over 10,000 regals. Under my system the tribute for such a faction would be around 2,000 regals, which is both realistic (who the f*ck had 10k to burn?) and reasonable (who the f*ck is going to pay 10k?) without being crippling to PvPers (its still worth as much or more than a new faction).
Thats just my rant + idea here. Take it as you will.