I'm not sure it would be cheaper than a PLB. Even if it successfully flew to a phone, how would it use it? Wouldn't it be better to have a built-in phone onboard? This incurs the cost of the phone electronics and service charges. It might be useful, not because of economy, but because you can use it in places where a PLB doesn't have a signal, i.e. caving, but I think your market is going to be very small indeed.
I'm not trying to be a nay-sayer; I'm just trying to be realistic. In any case, I hope your project goes well.
EDIT: Sorry, I misread your post. You didn't say the nearest phone, you said the nearest signal, so I realize you meant it would have an onboard phone. Never the less, I'd be surprised if you can manufacture, market, and deliver for less than a PLB.
Edited by 4evrplan (10/25/17 10:05 AM)