I only cheat to run tests..
However if you did not buy a single thing in game and sold everything but one gun.
It would amass quite quickly, basic charisma and barter gets you roughly half the asking price from traders.
On a normal run, where I buy ammo constantly, and keep most weapons, armour and misc as spares.
I run around 20'000 caps minimum as I hit
Narnia NV on a linear path.
Make this up again within a day or so more playing and looting around the city.
Take out the cost, and if I sold everything using the respawn glitch you could treble that easily.
I'm not going to say a million total, but I dare say at least one person has reached that at some point or will do.