I designed & built a prototype 2-wire system using a simple protocol about 10 years ago for a Freescale microcontroller design contest. At that time, the estimated single channel decoder BOM cost for the PCB and components was under $2 (not including the case & potting). I posted a document & video with more details of the design, see the links below.
I would like to contribute to the definition of an open standard and design. The initial goal would be to build enough units for widespread field testing to prove the design. The end goal would be for the major irrigation companies to adopt the open standard and integrate the decoder hardware in solenoids for residential use.