The total construction cost was around £70 but if you make them in bulk with four boxes to a full sheet of 9mm and 12mm ply, it should come down. I also added a roofing felt cap over the top to keep the rain off longer.
This is the timber all cut out and ready for assembly.
The assembled and painted bat house with a 30cm ruler for scale.
View with the bottom door open at the four internal chambers.
Total construction time was about three hours, excluding painting.