In Konquest mode, characters that appear with a puff of smoke will hand over a very large sum of Koins. For example, Motaro will give you 1200 Gold Koins.
In Konquest mode, many characters will ask you to do missions for them, usually fetch-type tasks. Completing these missions usually will result in a good sum of Koins.
In Konquest mode, use Punching as a tool to get different responses from characters. For example, Mocap in Chaosrealm will say "Ow my balls". Also, punching certain characters will make them drop Koins.
If you have already completed Konquest mode, unlocked the graves in the Krypt with everything you have collected, and there are no more Koins to get in Konquest, create a new profile. Get all the Koins you need. Then, start a two player versus game. Load both profiles and press R to wager the desired Koins. You now no longer have any use for the second profile. Delete it, and continue.