I'd like something similar to his first prototype: Battery powered if needed, and 10 minute refresh (or scheduled push). But the key is that it has to be sub $5

I'd like something similar to his first prototype: Battery powered if needed, and 10 minute refresh (or scheduled push). But the key is that it has to be sub $500 total cost, and work free or locally. If I need to pay for a service or the hardware is $1k then it's already not paying for its own utility.