I have gotten some sales like that in late game. But consider the options.
If I have an income of 3000 gold/turn-- and I have had in some games... 10% of that for luxury/entertainment for the cities is 300 per turn. Same for research. If I put 50% of that into research for 4 turns, that is 6000 gold. 3000 is half price.... and you can be doing something else with your gold, like other research, etc.
Happiness 150 gold/per turn is half of 10% of 3000.
Now, in early game...
I have a problem with AI trading. They want all you cash -- say 800 gold. But won't pay more than maybe 200.