Two Ways to Earn Money

The Short Way:
(12500G per try, but you need to sit in front of the TV)
What you need - beat Tower of Lost Souls

1. Choose level 60, and pick Xianghua or Kilik
2. As soon as the round starts, back up to the edge of the ring and do a move that leaves you grounded (Xianghua's 1A+B(hold), Kilik's 1B+K(hold)).
3. Roll around as needed, making sure that Algol's line of vision is perpendicular to the edge of the ring
4. Wait for Algol to ring himself out

If you're lucky, you can make 12500G in one minute.

The Long Way:
(10000G per try, but you don't need to sit in front of the TV)
What you need - a controller with autofire functionality

1. Set X (PS3) or A (Xbox 360) as the horizontal slash button (A, by arcade notation)
2. Choose Siegfried in Arcade Mode
3. Leave A turned on

Siegfried will continue to do A slashes, which will eventually beat story mode for 10000G. The cursor will still at Arcade Mode when you beat it, and the default character is always Siegfried.

It takes 10 to 15 minutes to beat the game this way, which means you'll make anywhere from 40000G to 60000G every hour.

Source: 2ch, Soul Calibur Forums