Trading techs and AI Money

In mid to late game how do other Civ's get money quickly?

Here's what happened to me and it seems strange. I traded a number of techs to a non aligned culture (the Scot's I think) for all the money they had on hand (something like 24000 for 5 years) I would think that this would bankrupt the civilization. But on the next turn instead that civilization had more that $100,000. So how did they do that?

Difficulty: Cakewalk
All 5 U.P. Civ's were active
Map was huge or gigantic...