Have you considered the new EF-S 10-18mm f/4-5.6 STM ultrawide zoom for crop? Its MSRP is US$299, so it's about half the price of the EF-S 10-22. Sorry, but due to the crop factor, you're not going to find an ultrawide for full frame that also performs as an ultrawide for crop, and definitely not one for $500 or less.

To me, this would be a far better choice and far more practical than the Rokinon 14/2.8, which will be only as wide as 21mm equivalency on a crop (not 16mm equivalency), suffers from wave distortion, and is a manual-focus/manual-aperture lens with no electronic communication to the camera body (practical upshot: you have to shoot in M or Av, you have to manually focus, manually set the aperture, use stop-down metering, and you'll have no EXIF lens information. And a focus confirmation chip probably won't work as well as you think it will--I've found it to be relatively useless for precision manual focusing--a better focus screen or Magic Lantern's focus peaking and Magic zoom features work far better for me).

Both the Tamron 17-50/2.8 and Tokina 11-16/2.8 lenses are for crop, not full frame (Tamron's Di II, Tokina's DX, and Sigma's DC designations are similar to Canon's EF-S: the lens is designed for a smaller sensor and will vignette on a full frame, although the 3rd party lenses may still mount on full frame bodies).

Ultrawide is the one place where it make sense to rebuy glass when you move to full frame. And there's no point in compromising on a main lens you plan to use extensively for the sake of a camera you don't yet have. Unless your full frame purchase plans are imminent, go for a crop ultrawide lens.