You will find it difficult to get an effective Gil sink, frankly.

I think that 25-75M mounts are enough, and with the frequency that they give them, there will always be a problem of... People who farm Gil will always farm said Gil, faster than what is possible to develop adequate assets to address. Similarly, there are still folk in the game with an absurd amount of Gil, to the point that these forms don't even tickle.

This posits the question... What level of player are you aiming at with this?