Great thread over at WuS about sub-$500 chronos. I second the notion that the Swiss Army chronos are killer and very inexpensive. Hamilton should be down in the $500 range as well.
There are some Swiss Army 9G 600 Chronograph going on ebay now for $450, they have the valjoux 7750 movement. You don’t often see that caliber movement at that low price. There is a massive thread in the affordable forum here talking about them.
Which chrono watch under 500$ – Watchuseek Watch Forums
I recently bought a nice FA-18 with valjoux 7750 on ebay at $490. The seller’s name is oleg47ny. You can check the listing http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110290470075