I'm just curious...Do you aleady own an XMON? there are cheaper talkback solution.
I' am using the System 5 mc internal mic into a small $50 mixer:
http://www.behringer.com/EN/Products/502.aspx
The output feed one of PT input. I use busses as send to headphones and Massey
talkback plugin for the auto talkback feature:
http://diamond.he.net/~smassey/plugin.html
If you need latch talkback, you bypass the plugin and use the System 5 MC mic aux input "on" switch.