I'm a bit puzzled as to why an invoice price or MSRP or any other pricing, beyond what a person actually pays for a vehicle, is that relevant. If my dealers invoice shows $100,000 but they give me 80% off, am I supposed to feel any better than just paying $20,000 for the car?
The issue of invoices/dealer costs/hold backs/rebates/floor plans/etc. can be beaten as long as anyone is willing to discuss, I just question to what end? In my opinion, the only thing that really matters to me is what my actual out-of-pocket costs are. Everything else seems to be smoke and mirrors.
I imagine that the biggest rub is that all of us, myself included, want to feel as if we got the best deal and that we somehow "beat" the dealer. I'm sure that several folks can regale us with stories of how they beat the dealer, and I'm sure that some have. Dealers that get beat a lot are probably not dealers very long. Good dealers make you feel like you beat them as they take your money anyway.
Whatever makes someone feel better as they go through the process of buying a car is certainly their prerogative, but it is all very subjective.
Good luck and take care.