In Fallout 3, a large amount of money would be about 4000-5000 caps. Now, I know that sounds a little odd to you as you're probably used to New Vegas' vast surplus of earned caps but the times in DC are rough. I noticed this too, when moving from Fallout 3 to Fallout New Vegas. I could hardly dream of obtaining the amount of caps I'd get in the Mojave when in DC. I thought I had it all and was living a life of luxury. Little did I know, I wasn't nearly as close to the fast life as I thought I was. I barely had enough to pass the credit check into the Strip.
As far as ammo and weaponry goes, the Mojave is filled with more updated technological weapons, with Fallout 3 staying obsolete. It's good this way though. Fallout 3's obsolete weapons is what differs itself from other games.
The Capital Wasteland, I think, there's more of a struggle to survive(Especially within that cap-obtaining). I was a level 3/4 one time and I remember trekking all the way to Tenpenny Tower, only to be informed that I didn't have enough caps to pay off the 100-cap entrance fee. I don't think there are many rewards in quests from DC, because face it, people just don't have the money. The Capital wastes obviously aren't as rich of an area that New Vegas is. And that's why I think there's differences in value from the two.
And come to think of it, there would not be enough caps in New Vegas to uphold in casinos, bars, etc if New Vegas was similar within the amount of cap-currency to Fallout 3. There's just no way. But Fallout 3 never had as many casinos/corporations as New Vegas had so there was really no point in DC having such a large amount of caps anyway.