Have you all noticed that during certain gun fights (ambushes/bosses) that there are ammo laying around? theoretically you can keep collecting it for lots of ammo & sell it for money. Granted the issue with bosses is survival, and time....
approx ammo respawn time = 25 seconds... approx ammo collected = 14 pistol approx ammo collected = 35 assault approx ammo collected = unknown approx ammo collected = unknown
Easiest & Safest location (so far) =
Graybox: During Mina's bonus challenge
At the end of the catwalk, from which you shoot Darcy, there is a respawning ammo crate that always gives 13 tranquilizer rounds. No bosses or enemies to deal with.
Approximate Respawn Time = 12 seconds... for 13 tranquilizer rounds that fetch considerably more money in the clearinghouse than any of the other infinitely respawning ammo caches.
This location is immediately available, safe, and will fetch you a lot more money in a lot less time.
Mission = Intercept Nasri the Arms Dealer, after defeating Nasri's guards, theres a shooting range off on the side where ammo will respawn. just camp & collect....
It'll take time....
Edit: Be aware that this method for hoarding money takes a lot of time. Considering the 25 seconds respawn rate, you can take ammo 144 times per hour. As it only spawns regular ammo, which sells for $20 a batch, one hour of this will give you only $2880.
During Intercept Surkov at US Embassy if you have the Improved Data Theft ability; You can hack the computer terminal by the security room and receive $3,000 then get killed and load the last checkpoint. This will put you right back before hacking said terminal, but this time the amount will double to $6,000. die again, and it will be $12,000. Each death will cause an additional doubling. This was tested up to $3,072,000. It is unknown if this also works on other computer terminals in the game, but theoretically you can be set for the entire rest of the game if you get to this mission as soon as possible and die 9-10 times (for a cash payout of around $3,000,000), mind, this only worked for the terminal, ammo, items, and cash piles were unaffected, and this was the only place where this was noticed, but it may have been due to the high initial amount that it stood out after the first accidental death (...and the next few...not so accidental...deaths)
This has been confirmed it also works on Assault Lazo Yacht. There is a terminal on the bottom level, shortly after you go down, right before you meet Lazo. Nets 10.000$ on easy.