economy in ro is affected by 3 thing: Gramp, Box, Update.
Gramp:
Every week, the monster inside Ti will be replace by another monster. Sometime, Gm will put monster with really good drop inside Ti. Example: this week current MidTi, which is Abysmal Knight. Normally, its card worth something around 40~50mil, but because of ti, you can see that it has drop to around 15~35mil this week. It is nice if you can rise a character, then keep it in ti range. If you manage to do that for all 3 ti, then you should be able to farm the ti if there is interesting monster around.
interesting fact: Abysknight market is kinda screw up because we recently had them in ti, and gm put them in before giving chance for their price to stabilize.
Box:
Most of the good gear only come from box. Faw, HBP, Skull Cap, etc. When they are around, you can get these gears for a cheap price. Basically, hoard a bunch of them, then resell later. Take note that there is a granny in eden2nd floor who accept certain gears from box and trade them with brownies point, which you can use to trade for box.
Update:
New episode will bring new change, or a new meta. Certain items will become more valuable, and some may die out. Basically, predict what will happen with new update (or episode) and test your understanding about the game.
Please read maintenance topic every week. Sometime, Gm will put in certain quest/event that you can use to get decent zeny. For example, during halloween, Gm give us candy quest. This candy is extremely useful in game. You can try to farm them, hoard a bunch, and sell them later when they remove that quest.
Take note that this is what affecting iro's economy, not how to make zeny.