I think it is too separate issues too with trade deals.
The more the AI like you the bigger a range it is will to accept. So it values embassies at 2gpt but will give you up to -75 on trade deals so it accepts 4gpt and 1gpt back. I don't think there is any good reason for the AI to do this, they should just only accept things for what they value them at, there is no need for a range. It makes selling resources one at a time best too which is unfortunate. (you an also tell if they secretly hate you doing this which is a bit gamey)
Then there is an issue that the AI just can't tell what a resource is worth. And it is so subjective, iron/horse sometimes is worth quite a bit but the AI will just take all of it leaving the player swimming in gold. Like CrazyG said you have way more gold from AIs than your own cities!
This has a knock on effect that makes all gold generating buildings like markets awful and buying things the default rather than sometimes. Rush buying every building and spaming emissaries into every nearby CS is a bit too easy.
There is an issue that it scales differently on different difficulties but taking all the AI's money on prince is probably still an issue, you might get less but it will hurt them more.