Invoice is invoice, and you can look that up here at Edmunds.
If you mean the actual price the dealer ultimately paid the factory for the car, then no. The dealer probably doesn't even know what that number is until the accountant does the books; the dealer would pay invoice, but may get bonuses, spiffs, special financing arrangements (in addition to holdback), free training, special ad packages or sales cruises.
I don't see anything wrong with your method, but it may be easier just to get the True Market Value from Edmunds and lowball it. Or use the Dealer Quote service here and see what kind of numbers you get.
If you do cell phones, you can use Edmunds Mobile Site at the out of town dealer. Or ask the salesperson if you can use their computer (I bet they'd love that, lol, but you never know - they may have PCs available in the service area). If not, find a public library.
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