I noticed random battle winnings had some large variations and just realized why. Repeating battles in the Coliseum results in winnings being reduced for that particular battle (1/2 for 2nd time, 1/3 for 3rd time, and so on) and this reduction carries over to the random battles, so for maximum winnings be sure the last fight you had in the Coliseum has a "1" by it indicating you fought once only. Repeating random battles will not result in reduced winnings, but the "reduce winnings variable" is set in the Coliseum and does affect random battle winnings, and it is reset to 100% by playing a new Coliseum battle (only needed if you repeated a battle in the Coliseum).
Ah, interesting. I don't remember if I intended that to happen or not. :)