Good luck with that. At $100 per 8GB of ECC RAM, that's $1mil in RAM, and with 4-slot machines you'll need 3,200 boxes. 1,600 boxes if you go for dual-CPU machines with 8-slot.
First, 8GB ECC sticks are actually a bit under $100... retail. You're not going to buy almost 13,000 sticks of RAM retail, are you?
Second, server motherboards are not limited to a mere 8 slots. 16's are widely available, even 32's can be found at retail, and there are more options out there than what Newegg or whatever you're shopping at has.