Economic Domination

[Skott]

Usually you have to give them at least 10,000, which is fine china if i remember correctly, to get them from hostile to very friendly. If you cant afford that all at once (one turn) give them 5,000 twice (2 turns) or 2,500 four times over four turns. You get the picture I'm sure. Sometimes I find invading and conquering the nation is easier and more satisfying than buying their friendship.

[Quillan]

It looks to me as if a "State Gift" gives a +100 to relation status no matter if you give them the 2500 sovereign china or the 10000 sovereign jewelry gift. Also, the diplomacy modifier degrades over time, and giving another state gift just brings it back up to the +100 level rather than giving an additional bonus.