The cheapest would be an IR camera, a small display, and a lens. Here's a 3D printed kit, with links to parts and communications channels for makers: https://www.tindie.com/products/justcallmekoko/bpnvg-3d-prin...
A step up from that would be the lower end commercial offerings like the Sionyx Aurora: https://www.sionyx.com/collections/aurora-night-vision-camer...
Both of the above really require active illumination to be effective. As long as you're not worried about other people with night vision, that's not a problem.
Above that... it gets expensive and complicated very quickly.