You can get a standalone (cheap) Geiger counter like the GQ GMC-300S on Amazon (B0B541D433) for ~$50. If you're interested in monitoring radiation and have a higher budget to play with, I highly suggest stepping up to the Radiacode 103G (~$600) or the Radeye B20-ER (~$3000). The Radiacode can only detect gamma/x-ray, but is a spectrometer so it can determine the energy level of the radiation. The Radeye can do the full suite of γ α β, and can provide energy-compensated dose rates when its gamma filter is in place.

There are/were Arduino-compatible GM tube kits from China for $35 USD.