I'm not usually a big fan of cutting corners like this, but for you guys I'll make an exception. Even after beating the game and saving as much money as posible, you usually don't have near enough to upgrade your weapons much let alone buy new guns.
The rotten egg "glitch" (it's not a glitch, they allow you to do this for a reason) requires you to find a rotten egg then give it to yourself repeatedly until you have as many as you want. Each egg is worth $2000 so it doesn't take long to get what you need.
All you do is find a rotten egg, start a splitscreen game, then give your egg/eggs to your partner. Once that's done exit the game while only saving the profile for the player receiving the eggs. Do not save the profile for the person giving the eggs, this way when you restart the game both of you will have eggs. The person receiving the eggs then gives the eggs back to the person giving the eggs doubling the amount of eggs you started with. Once all of your boxes are filled with eggs, all you have to do is give the eggs to your partner, then before restarting the game the person receiving the eggs simply removes them from their inventory. There are alot of videos on YouTube showing you how to do this. The only problem is finding the eggs in the first place, as they are rare and are randomly generated.
If you would like, we can get into a game together (360 only) and I will give you 45 eggs and explain the process to you to get you started, this will save you alot of time. If you haven't added me as a friend, add me then contact me so we can set up a time to do it.
To put it into perspective, each time you you give the eggs you are making $90,000. You can make a million in about 10 minutes this way.
