Have you considered a cheaper route i.e. sound only?
There are simple 2 way boxes which use an FM signal on the mains wiring.
The key switch in Baby's room is locked on, and you will be able to hear every breath if the volume is up that far.
In my time I found that any gulp or sneeze from the black box, and my ears pricked up immediately.
As our children got a little older the box was disused, but if there was a hefty thump at any time from another room we waited, held our breath, and were reassured by a good yell!