I've finished the board layout and put together a bill of materials. It looks like my estimated cost was fairly close to the the mark.
I don't yet know what the board itself will cost so am estimating it based on a similar sized, low volume board (i.e. BX24-AHT).
For those buying the parts from Mouser and doing all of the assembly, it will cost about $35-40 plus shipping. This excludes the ZX-24 chip ($60 after March 1, $45 currently ), an enclosure (Polycase LP31F), the xPort Ethernet module (Mouser $49) and cables. This requires soldering of 4 SMT chips and several SMT resistors & capacitors (805 size) plus through hole connectors and capacitors.
If there's sufficient interest, fully assembled boards could probably sell for $50-60 including the enclosure but excluding the ZX-24, xPort & cables. Users can jusp plug-in the ZX-24 but the xPort cannot be socketed and will always require soldering - it needs the copper on the board for a heatsink.
So it will vary from about $80 to about $160 depending on whether you assemble it, when you buy the ZX-24 and whether you buy the xPort.
If you want X-10 RF input, it will require soldering a jumper on the main board (+5V power for the MR26) and replacing the serial cable on the MR26 which will also involve soldering. There is a replacement PIC I did a couple of years ago for the MR26 that allows it to handle X-10 security codes. This requires soldering an SOIC-8 SMT chip.
If you add the xPort, it web-enables anything connected to Rosetta.