On my first playthrough I ended up with about 50,000 caps. It actually gets much easier to amass them after a certain point, especially if you get your repair skill and barter skills up to 100. Early on you have to use caps to buy stimpacks, get repairs, buy ammo, but eventually you stock up so much ammo and medkits you start selling it. For example I never used the 5mm rounds, or flamer ammunition, but whenever you find a mini-gun or flamer there's usually plenty of ammo so it's an easy way to make money, and once you've found at least 100 stimpacks you realize you're never going to use that many so I sold any over 20 that I found. They just accumulated over time with selling things I didn't need, and the pre-war books quest helped boost it a fair amount. Also it helps to realize that most of what you need can be found if you look for it. Spare stimpacks, ammo, rad-x, rad-away, weapons and armor for repairs, etc... can be found all over, especially if you're anal about searching every filing cabinet, desk, or bathroom stall you see. After level 5 there's not much of a point of buying items.