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.