Wow, if you can get an '04 Insight for $500 over invoice, that would be a very good price at this time of the year.
As far as I know, at least in metro-ATL, the Honda dealers will only order 2004 Insights if the customer puts down a non-refundable deposit. In other words, there will be pretty much no room to bargain.

Only if someone orders one, changes their mind and a dealer gets stuck with one, will there be any sort of a discount.