The last time I saw a "cheap" thermal imaging camera it cost about 600 pounds. But there's now an FLIR camera available for IOS
UK vendor Rapid electronics, there may be cheaper alternatives
I won't be buying one for myself, but I would have been all over it before I retired.
update - looking at this more carefully, it seems to be designed to fit onto the iPhone 5 and 5s rather than generic connectivity, which means that if you have an older iPhone or an iPad you're probably out of luck. I was originally told there was an android version but this appears to be untrue, sorry about that. The Seek Thermal mentioned in comments appears to be for iPads with lightning connectors as well as iPhones, and there really is an Android version, but it's not as versatile in other respects, e.g. it doesn't take a visible light picture as well as IR, also the cheapest I'm seeing it in the UK is on ebay at £269.