There are other technologies out there, too. You don't need full wifi for most bot stuff... and if you both had tons of bots and were sending lots of data around, you'd have problems no matter what. As for the price difference, I think a lot of it is economy of scale, but I'm not sure.
Some of your other options:
-Mesh network of Axon Motes ($70 each, wireless+mcu)
-Mesh network with Series 2 Xbees (aka zigbee protocol) (~$30/bot)
-Bluetooth (~$50 for class 1... not so great a deal...)
-Pololu's Wixel ($20/board, wireless + very simple MCU)
From what I've seen, swarms of bots are mostly fantasy or university research project (which have $$$)