Unfortunately they aren't cheap, I'm getting a custom winch built that is upwards of $3,000. I've used a home made operation and it gave me more problems than anything, but did end up with some shots out of it. I'd suggest first maxing out your speed abilities with the bungee. Its a lot of work but if you get 3-4 people to pull that thing back you can get decent results. Watch "The Arena" by Technine, I think its safe to say those guys max out what you can do with a bungee. Its on iTunes for less than $10.
From my experience the riders mostly like the winch more because it allows you to carry your speed and have time to setup, where the bungee is super quick and barely any time for setup. Hope some of this rambling helps!