How expensive is something like this?

You already have your smartphone, most probably: https://phys.org/news/2014-06-smartphone-detector-app-positi...

Smartphone cameras are sensitive to radiation (radiation produces additional spurious reading in camera sensors - additional noise over thermal one) and it is possible to use these cameras to more or less accurately measure radiation levels.

Couldn't someone make an app?

I looked into this once because I live near a reactor. There's a wide range of quality, but these restored classic ones have a good reputation for $250.

http://www.prepguard.com/prepguard-geiger-counter-radiation-...

$130 or less, shipped.

https://soeks-usa.com/collections/radiation-detectors/produc...

you can build your own https://www.sparkfun.com/products/14209

That's cool, and cheap at 70. But it doesn't detect alpha radiation. Unless I missed something.

You can shield yourself from alpha with two sheets of paper.

You can still inhale and swallow it, internally it is more harmful than beta or gamma.

What about washable face masks?

You can inhale those too but it's much harder.

Hardyhar

So they said in about the paper hat conspiracy.