Spend the money on a smart trainer instead. A smart trainer used religiously with a good app like TrainerRoad will boost your FTP like nothing other.
You only need one gear on a smart trainer.
You also want to make sure your training body position is nearly identical to your actual riding position, as aggressive as you can get away with. Best way to do this is to use the same bike. I made the mistake of making my trainer bike too comfortable (upright) during my first year of indoor training, and was unable to deliver the same power on the road in a more aero position.