Now I know this might be getting repetitive, but regarding those additional bonuses for reaching new culture levels that somebody will surely implement someday, I thought about it some more and came to the conclusion that instead of a city upkeep reduction a bonus to trade route yield would be more fitting, since culture is supposed to reward "tall" empires. Also minuscule bonuses to Great People Birth Rate like +10% serve absolutely no point, as any city that produces more than 10 GPP a turn is practically guaranteed to have a lot of culture already, so let me revise my proposal one more time:
10 Culture - Fledgling: +20% Defense, No bonus apart from that as ability to work a good part of the BFC is reward enough for such a small investment
100 Culture - Developing: +20% Defense, +1 Happiness ("We are so sophisticated!")
1.000 Culture - Refined: +20% Defense, +40% Great People Birth Rate, +20% Trade Route Yield, +1 Happiness ("We are so sophisticated!")
10.000 Culture - Influential: +20% Defense, +60% Great People Birth Rate, +30% Trade Route Yield, +1 Happiness ("We are so sophisticated!")
50.000 Culture - Legendary: +20% Defense, +100% Great People Birth Rate, +50% Trade Route Yield, +1 Happiness ("We are so sophisticated!")
Incidentally the culture values required for Refined and Influential should be doubled, so it's less of a leap from Influential to Legendary.