This is one of those strange problems when looking at it from the outside. When looking at the cost of what you're already doing (providing a
7942 phone for home use, please some sort of bridge device), and the cost of a power supply is an issue? You're several hundred bucks into this, and the power supplies are a fraction of what you've already got in this.
The 7942 is an older model phone. It's likely that the device you have is made to work with the newer devices, but I'm not familiar with that specific device you have.
If you really want to do this cheap, scrap all that and put IP Communicator on the computer they are using. Then you don't need any of those things and save the cost of the 7942 and the bridge.