Gathering gold/money can be a pain, use this Resident Evil 5 gold farming guide and you can be making 40.000 in one chapter. The basic rule: Find gems or items which you can then sell.
Look out for barrels and stashes
Yes, you heard right, make sure to open every barrel, especially those and dead-ends. Those are the ones containing cash or gems. On top of that dead-ends are the locations where you'll be finding new weapon upgrades. Look out and you'll find loads.
Best farming chapter/level
I discovered that chapter 4-1 seems to be best for farming. Lights for example when attacked will drop a topaz/gem. Furthermore whenever you'll be activating a statue, you'll be finding further gems. The last boss in that chapter alone drops an item which is worth 10000 when sold.
Collecting eggs
In case you find a golden-egg you'll be able to sell it for 1000. I personally never collect other than golden eggs as I prefer having inventory space for other stuff. It's your call. Brown eggs should be thrown away, white ones can be eaten, the golden ones are for sale.
Duping Items
You can also
dupe your items in RE5. That's probably the quickest way as you simply duplicate your items all over. However, for me it ruined the fun factors as you no longer need to farm treasures and money.