Sorry for the late reply, price per PCIE slot on the motherboard, or how much does it cost per graphics card on the system. If you have a $100 setup that holds two cards, then that's $50/slot, but if you can get a $100 setup that holds 4, then you will be at $25/slot.

Every time you have to buy another motherboard to run more cards, you also have to buy another CPU, RAM, and at least 1 PSU.