It's already just $500/kw for reputable Chinese cells. Assume you get on average 5 hours of usable sunlight a day and electricity is 10 cents/kwh. The 1 kw panel will give you 5 * 365 * .10 = $182.50 worth of electricity. So that gives you a $182.50/$500 * 100 = 36.5% annual return on your investment.

(I realize I'm simplifying here, storage blah blah). The elements declining slowest in cost are installation and regulatory hurdle jumping.