You should check the "prices paid" forums for each of those vehicles, but for a Camry or Impala, you are probably talking about invoice + $50-$100 - rebates.
What you want to do, however, is know what invoice is and know what rebates are and present your offer to the dealer as a price rather than "I'll pay $x over invoice." Just work with the price. If you try any other avenue, you are creating an opening where the dealer can play with you ... like showing you a phony invoice. And don't negotiate a price before rebates because that will just confuse things.
So, for instance, if invoice on a Camry LE is $19k and there are $1k in rebates, then you simply state, "I am offering $18k for a Camry LE," and work from that point. If you want to leave room for negotiating, then offer $500 below invoice minus rebates ($17,500 in my example above).