I don't if this has been found yet, but thought i would shout it out.
In the area where you first get the H&K MG you can go in get the items, quit game, save equipment status, and the gun and items will replicate to get two guns. easy way to make money.

Nice find but It does not net you much money , Only stealing treasures and selling them for the sake of humanity , freedom and a better future can make you rich !!!

Best way to make money so far seems to be farming the Lickers at the start of 5-1.

I wouldn't call it a bug. I've been farming the first few levels for ammo and treasure, upgrading my guns whilst I'm at it, generally the guns only make you about 200 gold, whereas the treasures are usually 1000g at a minimum.

"farming the Lickers"

Actually I thought of this trick the first time I saw RE5 save charater status and check-point-based level progress separately. I believe that is actually a deliberate desigh decision. I think any one with some understanding of how games track player status and progress could think of this, not just me, let alone the people who made RE5. I mean, they design games for a living after all......

As a designer(of hardware or software), you must always assume that some of the people who buy your products are retards.......
This saving system mechanism in RE5 is to make sure that people who spend all their in-game money on the wrong thing or waste too much ammo in previous level they've played are not stucked in the next level.
Nowadays, most of the games avoid the old-day rule of forcing players to start a new game as a punishment.

I believe that's also why Dead Space is implanted with unlimited stasis supply stations and oxygen and stasis refill cheat codes, instead of forcing players to rely solely on recharge items, even though Dead Space is some what designed as a survival horror, the genre that build its tension upon strategic use of limited supply.
Since RE5 leans towards co-op shooter genre and doesn't quite treat itself as a survival horror, it's natural for me to see RE5 includes such saving system mechanism to prevent trigger itching shooter players from stuck and to off-set AI partner using up too much ammo.