Compost is a great idea, much cheaper than buying soil every time from your local nursery.

I too myself was interested so I tried it out and juiced up a whole tray (you know those black garden trays) of wheat grass a while back. It made a lot of juice!! It was around 10-12 ounces of wheatgrass juice, I had to freeze some into ice cubes because I couldn't drink that much and didn't want it to go to waste.