Costco/Invoice/Fleet Manager

I'm looking at purchasing either a new Chevy Suburban or possibly upgrading to the Cadillac Escalade. Costco says that if you contact the dealer they have special rates that are basically selling the vehicle at "invoice" and you deal with a dealer's "Fleet Manager." For the Cadillac it is "invoice plus $300." Is "invoice" a good deal or just another scam? The difference in the MSRP and "invoice" of the Escalade was less than $4,000 which seems low to me. Seems to me that dealers never sell cars at MSRP so at best they'd be making $1-2,000 on an expensive vehicle like this. That doesn't seem right to me. Seems like dealers would make a lot more on such a vehicle and there's be more room to haggle. Is invoice really as the salesman says "the best I can possibly do" ? Would I do better not to go through Costco?

*shrug* Last car I bought, I got for $1200 under invoice. After dealer tax and all that, I got it for just about the advertised price. I'd say buy it from a Cadillac dealership.