> One possible solution is to network over wireless, but this is very
> expensive when I have 5 or 6 units to interconnect.

MRF24J40-I/ML is only US$3.24

MRF24J40 is a complete IEEE 802.15.4 radio and operates in the 2.4GHz
band. The MRF24J40 supports ZigBee=99, MiWi=99 protocols and proprietary
protocols to provide an ideal solution for wireless sensor networks,
home automation, building automation and consumer applications.

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=3Den027752

Good Luck

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