Maybe we're looking at this too hard to balance it versus artifacts. What if improving a Landmark meant you got instant GAP and WLTKD to whatever plot belonged to the city? If we're still not satisfied, it can also grant one of the 4 random type of generic great people: an inspiration for your great scientist, engineers, merchants, or civil servants.
Don't forget that you also build landmarks in other civilization's territory to deny them artifact bonuses.
http://civ-5-cbp.wikia.com/wiki/Landmark
Not a huge fan of that tbh.
How about just make all landmarks the same strength (probably drop the whole era-scaling we have on them in favor of certain increases on certain techs, like most other improvements work) and then buff/nerf them to a point where people can't agree what the best choice would be.
It would be nice if there was some distinct difference in yield-type between the Landmark and the Artifact, just so that the choice would feel more like a choice and less like a calculation, but the landmark is the culture improvement and I really don't want to move away from that.