There are a few ways to get lots of money in RE5, but I found one way that's better than the others. In Chapter 4-1 The Caves, you can go through it and pick up tons of gold and tons of gems. You can get over 15,000 gold each run in a short time and have fun killing enemies too. Makes it alot easier if you have an unlimited ammo gun already like a magnum. But if you don't want to have any enemy encounters, you can do 3-1 The Marshlands.

Chapter 4-1
1.  Run through pass the spiders, then up the ladder.
2.  Pass through the spiders again, before going through the door, pick up the 2 gems on the ground and one in the fire pot.
3.  Just go the way you're suppose to go and you'll see a large stone box that contains gold/gems. There's two of these boxes to find.
4.  The third box is just a trap, which you'll fall down, go head and open it.
5.  You'll end up in the middle of a cross section, stay in the middle.
6.  Kill the enemies, then open up the 3 chests containing a large gem, each worth 2,000 gold.
7.  Next, go kill the enemies in the large room, then shoot down all the vases.
8.  Go collect all the gold.
9.  There's no need to go further, just quit and save.
10. Repeat this stage for maximum gold and fun.