The thing is that with a cropped sensor like the 50D, the fisheye effect produced by the lens has to be very powerful because the cropped sensor is only imaging the "center" of the image circle produced by the lens. Therefore, the Canon 15mm fisheye produces only a very slight fisheye effect on the 50D. Not to mention it's $500+ price tag.
On my Rebel XT (also cropped sensor), I have the Peleng 8mm fisheye. It has no electrical interconnects with the body so you have to turn 2 rings to set the aperture and focus manually, but the image quality is really excellent and it produces a strong fisheye effect. It's only real downside is bad lens flare. I got a used one from ebay for $300. Honestly, it's your best bet for budget fisheye. Anything cheaper and you're basically going to be looking at sh*t image quality.