Today i went hunting in SlowApes in the Mercury server and fouind that there might be a pattern for met drop rates.
First met = 220 kills
Second met = 420 kills
Third met = 540 kills
Fourth met = 960 kills
Fifth met = 1190 kills
Please keep in mind that those are rounded down buy 1-5 kills. So that seems random, but i'm not sure if it is. It does seem like some kind of pattern. That for each met you get, it will be harder for the next one to drop.
Thats 100 more for second met, 120 more for third met 420 more for the fourth and 330 more for the fifth. From that pattern it looks like the met drop rate to kills ratio (if it is a ration) goes down. I'll test this further and get back to you. Anymore info would be great!