Keep in mind the 1.3x crop, and you need to think about how you will eventually fill out your lens kit, so that your first lens will fit in.
For example, will you eventually go with something like 24 / 35 / 75? Or maybe 21 / 28 / 50 / 90? I recommend that you think this through, and then pick a good lens that fits into your longer term plans.
In the $500 or less range, I would recommend that you look at the CV lenses. They are very good value for the money.
The CV 21 f4, CV 35 Nokton f1.4, and CV 75 f2.5 are all very nice lenses. I haven't used the CV 50mm lenses, but I'm sure there are some good picks in there as well.