RS485 can be made rather cheap, it's just a single cheap IC to convert the serial from Arduino to the RS485 signals + a couple of resistors.
Also RS485 can handle both half and full duplex.
RS485 can handle much longer distances than 100 feet, but of course it's not a wireles system, you need a cable, this could be done with ordinary cat 5 network cable.
Here are some links to arduino + RS485 info:
http://gdallaire.net/blog/?p=39
EDIT:
Implementing full duplex where every node can talk to every other node is fairly complex. Because all the nodes share the medium (cable) you would need to implement a collision detection scheme, which is not exactly easy.
Half duplex with one node as a dedicated master that initiates all communication and a bunch of slave nodes that only respond to "calls" from the master is much easier, and requires only 2 wires in the cable. Full duplex requires 4 wires.