I did 3 experiments to find the best monster to kill. The conclusion is that having both Bally and Rockster brings in the most money and RP.
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Bally only:
start:
1.13B 2.24M
~30 minutes later~
end:
54.74B 2.65M
difference:
53.61B 0.41M
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Rockster only:
start:
54.74B 2.65M
~30 minutes later~
end:
104.49B 3.11M
difference:
49.75B 0.46M
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Both Bally and Rockster:
start:
104.49B 3.11M
~30 minutes later~
end:
178.38B 3.71M
difference:
73.89B 0.60M
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There is no reason to go above 100 health, 30 speed and 1 Max count for Rockster because the benefits are tiny while the drawbacks are huge.
There's also no reason to go above ~5.5 health and ~445 speed for Bally as the benefits are also tiny while the drawbacks can be huge in both wasting money to increase and losing out on income when they stop being killed instantly.