invoice - holdback (2~3%) - cash rebate => closer to dealer's real cost.
They make more money when you buy ANYTHING with the finance guy (warranty, fabric guard, underpan coating, maintenance contract, etc.)
In general, it does not matter what the real cost to them is, it is what kind of deals are possible in your area that really matters. Just send e-mails to all your nearby dealers (internet managers) and ask for a quote. Let them compete a bit, and pick the best among all.
Oh, say NO to anything the finance guy wants to sell you.