They run on a bus, which is extensible indefinitely, at least in theory. The bus can be connected with stacked headers, or via a jumper cable, so there's no capacity limit physically either.
Maybe you should repeat your first years electronics course, and the influence of capacitive bus loading and line inductance on speed. Please don't forget that you have only a limited driver strength. I bet just jumper wires with many connectors inbetween and no termination will give intresting results in signal integrity.
Additionally, I'd like to know where these bulk FPGA discounts are from, I've yet to find a distributor who will give more than a pittance of a discount on large orders.
Maybe you could ask ztex for that information. In the past he had a license production offer.