FNV economy is the way it is because of an oversight/bug.
At some point someone at Obsidian just decided to change (in an official game patch) the values of certain stuff and make it stupidly expensive, while everything else remained the same. It was almost entirely centred around armor, some of the weapons (not even 1/4 of them) and weapon mods (which some were clearly setup for the original economy, before the change), so some weapon mods have the old price and are really cheap, while the patched ones price is increased by a lot. Which makes no sense and feels quite off.
And in the process of changing the value of some things, they ended up breaking vendor repair, the economy and any sense of difficulty in maintaining a decent amount of caps.
Here's one quick (or is it?) example of something broken in the price of things in FNV:
Regular Combat Armor has a base cost of 6500 caps, Van Graff combat armor has the same base cost of 6500 caps. But the thing is that the Van Graff armor offers more protection (DT 16 vs DT 15). The rest of the stats are exactly the same in both armors.
Now let's take a look at the Van Graff combat armor (base cost 6500 caps) and the Combat armor, reinforced (base cost 8000). The reinforced armor costs 1500 more caps and only offers +1DT over the Van Graff armor. So it's the same situation as with the Van Graff's and regular Combat Armor, but in this case 1DT is worth more than 1000 caps, while in the previous case it's worth nothing.
To make things even more mind-blowing, let's take a look at the C.A. Reinforced and the US Army Combat Armor. The Reinforced one has a base cost of 8000 caps, and the US Army's one has a base cost of 8500 caps. But the US Army's one offers 1 more DT (18DT) than the Reinforced one (17DT), the US Army's one also gives a +3 bonus to the Guns Skill and a +2 bonus to the Survival skill. But how come the base cost is only 500 caps more than the Reinforced armor, when the the base cost of the Reinforced one is 1500 more than the Van Graff's armor?
And here's more weirdness around the combat armors value in FNV:
The Combat Armor, Reinforced Mark 2 has the exact same base cost as the Combat Armor, Reinforced... Their base value is 8000 caps for both... The only difference is that the Mark 2 gives 2 more DT... But if you have an armor giving 2 more DT, why does it cost the same? (all the other armor parameters are exactly the same)...
This was just economy problems with some of the Combat Armors... Now imagine the amount of broken stuff in the entire game... But to generalize a bit, some stuff (that shouldn't be) is too cheap, other stuff (that shouldn't be) is too expensive, some stuff has the same value (when it shouldn't) and it's super easy to be swimming in caps just playing the game normally.
I also just want to mention that a suit of T-51b Power armor (25DT +1STR +25 RAD resistance) has a base value of 5200 caps and a Remnants Power Armor (28DT +1STR +15 RAD resistance) has a base value of 6500... The best armor in the game costs the same as a measly Combat Armor...
The fan-favorite armor, Advanced Riot Gear (21DT +1END +10 Explosives Skill) has a base cost of 8494. Only 494 caps more than a Combat Armor, reinforced which only has 17DT and no other bonuses...
There is plenty of evidence of how someone at Obsidian messed up the economy of the game in a late FNV patch, by changing the value of some things but not accounting for these changes in regard to the rest of the game's economy. It's almost impossible to use merchants to repair some equipment by how expensive it becomes, it also makes no sense that a merchant asks for thousands of caps to repair some weapon when the player can pay a few caps for a weapon repair kit and repair it to a higher durability using that.
And then we enter a much bigger problem which is having two different game economies in TTW.
FNV's broken economy is not compatible with FO3's economy. This means that a player only has to go to the Mojave, grab some starting equipment in Goodsprings, sell it and go back to the Capital Wasteland with more caps than it would make in this side of TTW by playing for a few hours. It would also make it almost impossible to be able to sell FNV equipment in the Capital Wasteland because merchants do not have enough caps there.
It's easy to say that "but all you need to do is to increase FO3's merchants caps", but this would break the economy there, then we would have to totally remake FO3's economy to be more like FNV one, but as explained before, FNV's economy is all over the place and totally messed up due to them breaking it on some patch and never fixing it. It's better to fix the broken economy than to break the better one already in place in TTW. FO3's economy is not perfect either, but at least is way more consistent, better implemented and feels much better compared to carrying around tens of thousands of caps in a wasteland where everyone is supposed to be broke.
There are other issues with FNV's economy, that I barely touched or didn't mention at all, but I already made a wall of text and don't want to spend more time on this. I hope this post helps understand why we had to balance the economy in TTW.