Would be nice to be able to counter with, "I can't afford to just give that stuff away. But perhaps I could sell it to you for _________."
When it happens early or mid-game with a Civ I could use an extra bright green modifier with, I sometimes say yes if the request is modest enough (later it won't be so modest...), and when they want to renew the gift, I have it lowered to 1 gpt, which they "generously accept". That becomes a diplo bonus for fairly cheap.
The other advantage to this is that once it becomes a deal (after the first return), then the AI can beg anew. So that let you get the modifier for really cheap, and you don't lose it for refusing their other requests for help (I "bought" the modifier from Hiawatha for 1 gpt in one game and we kept it for the whole game; his further requests were more in the order of 100 gpt...).
After a few renewals of the help, if they really like you, they sometime accept to return the favor, and thus the modifier becomes free.
As for luxuries, when they're out of gold and come to beg and you accept, then, once that gift period is over, they generally like you very much and agree to a generous deal. My last one was renegotiated a few turns ago and after getting it free for a cycle after she came begging, Maria I offered me 106 gold + 7 gpt + OB for one Gem (that's Marathon).
Those requests are in a good part scams (the player needs to know there's no diplo hit for refusing those requests from declared Friends...), but they do offer some trade and diplo advantages if you can afford the lost gpt or the -4 happiness.
