This is a good item for a low level thief before they get Fang/Spike necklace, and maybe to macro in for stealing. Drop rate is very very low, but this may be the only factor in it's high price. Certainly not a necessary item by any means, and probably not worth the 150k or more it costs on most servers.
Rabbit Charm is dropped by Jaggedy-Eared Jack [NM]. Located in West Ronfaure G-9 to H-9.
Level: 9-10
Lottery Spawn from Forst Hare
Soloable at around 10~12
Pop is about 1 hour to 1 hour 30mins.
I agree with Yourshadow. Though you might even want to pick up a Feather Collar plus 1 if you can. Rabbit Charm definitely is good. Its best purpose is for your AF dagger, getting your Testimony, and for your Maat fight (assuming you steal instead of assault).
Besides all of those obvious use's. There is another use it can serve, and that it using it for Stealing currency in Dynamis every bit of +steal helps.
Its come down a lot, used to be 2 mil when I got the drop. I'm not sure if its because of the deflation or if its from a stealing nerf, but the reason this was always so expensive was higher level thf's getting as much +steal as they could for their Maat fight (successful steal = instant win).
Once you hit spike necklace level all this is good for is steal.