have you ever seen a "+1, you gave us a gift"? NOT the "you helped us", as this is a different situation. if someone can find coding that says different, i'll be happy to admit i'm wrong, but i've never seen anything to suggest that there's anything except the "fair and forthright" bonus you get for trade.
and yes, 30 gpt is kind of out there, but the "big civ on the other continent" has given me 27 for stone. and pantastic, if you're getting every gold per turn they have, and they're willing to renegotiate to give you that 1 more gpt that they've just got, i'd say that they're putting quite a high value on it. an extra health or happy is great for your cities, i never said to sell resources you use, i said i sell copper and stone when i get gunpowder. if you're hitting the happy limit, keep the copper, but since you only get the 1 happy (w/forge), it's half as valuable to you as another luxury resource, which can generally bring 2 happies with the proper building. but the AI seems to value it doubly, only seeing it as a "strategic resource", which it values more than luxury and health resources. at the very least, you can certainly afford to sell any stone or marble, as well as any doubles. if you're not getting enough resources to sell them, then try swallowing another civ early on, the additional capital site, as well as the land between your starting position and theirs will usually have enough resources for two.
edit:
one more thing, you can almost always trade, for example, my copper for your dyes and 5 gpt. the AI definitly overvalues this obsolete strategic resource. just cancel the deal if when you want to build the statue of liberty.
edit 1 more time... alright, i'm pretty sure the 27 gpt was on a mod, but i've definitly had 15 on vanilla