First, it is a pain in the rear, but go to BI(Bird Island) and pick up all the cash you can find. It'll take an hour or so, but you'll surround yourself with about a mill in your inventory and bank at once.
Next, you hit the markets! check all the prices there, and buy Meteors from people who offer the lowest prices. MAKE SURE you check ALL the prices of meteors, to see what the max and the minimum prices are for them.
After you buy some meteors for the cheapest prices you can buy them, go ahead and sell them at the medium price. Usually you could make 10-50k from each meteor in doing this. Keep repeating until you get a few mil.
Now is the tricky part! Buy Meteorscrolls at the cheapest prices, then sell them at the Medium price. Keep doing this until you get 50Mil
Once you reach 50Mil(The price of DragonBalls as of January 21, 2008 in Wonders-Colossius Rhodes server) Buy a DragonBall at 50Mil or less, then sell that same Dragonball for 52Mil or higher. This is how you can earn cash :)
All of this may take a few days, so don't expect anything at an instant.