I think it's about $120.
It's cheaper if using the regular tail assembly but I used the CNC version(which already cost $69 on its own).
The kit from freestyle-hobby is cheaper but it used the stock maingear, I use the T-rex gear so I can have more options on the pinion gear. I also manage to use one way clutch. The two main gears nead to be modified and an idler pulley bracket needs to be fabricated