Do consider the crop factor of your 550D - 28mm will give Field of View equivalent to 45mm - that's nearly a 'normal view'. Take your kit lens, compare with the focal length and field of view you get there at about 28mm. Not that wide, right? The cheapest wide angle is still your kit lens used at 18mm. Everything else will cost much more money.
Erm .. still your kit lens is wider with 18mm. Do you understand the focal length? If you want a lens with a wider field of view then your focal length must be shorter. That's the reason why people suggest the Sigma 10-20.
You already have a kit lens? What's wrong with using the wide end of that (18mm)?
Wide angle adapters are extremely poor quality (flare, distortion etc). Any good wider-than-18mm lens is over your $500 budget (17-85 excluded because the 1mm diff is hardly noticeable).
I have seen the Sigma 10-20 going for close to your budget (ard $550 IIRC) -- perhaps you can post a WTB ad and see if anyone will let it go within your budget.
But really, if money is that tight, it might be a better I idea to wait till you have more cash. At $800-900, you will have a much wider range of options.