What is the cheapest camera I can plug external microphone to?
Do you own a high-end smartphone? You can plug-in pretty much any microphone and have very decent audio quality. Some Android phones even support usb mics.
So you probably need a cheap adapter for standard stereo or mono mics to work with the 3 channel type headphone+mic jacks in modern phones. Something like this: https://www.amazon.com/StarTech-com-headsets-separate-headphone-microphone/dp/B004SP0WAQ/

It then only depends on the video quality of your phone. If that is satisfactory just get some sort of tripod adapter or other type of stand for phones and you're good to go. Definitely the cheapest solution if you already own a fitting phone.

If your phone is not an option I recommend you go with a webcam and USB microphone attached to your PC or Laptop. Most recording softwares have the option to choose a different audio source than your video recording device.
You can get a decent quality webcam for 20-30€ and a decent quality USB mic that can capture room audio for 15-20€.