Might be able to talk to local Lowe’s / Home Depot and see what they can do. Given the cause they might be willing to donate or discount some materials. Also might look into getting “culled” material. If you get along with the guys/ gals in the lumber department they have authority to mark materials as culled even if it has minor defects. Might have to get extra material but depending on how much the culled discount is (75% off at my Lowes) It could still lead to good savings.
If someone can access the material I will be glad to host several volunteers in my shop to make the boxes assembly line style. I live in the Indian Trail area so it would be easy to deliver them.
FWIW, I built my couple of wood duck boxes of Western Red Cedar. It's been a while so don't remember where I purchased it or how much it cost. I suspect around $4 a bd foot so probably more than what has been mentioned above.