I really struggled to find a cheap camera with 3.5mm microphone input, but eventually got the Kodak Zi8 with a PA80 adapter to record the audio.
Pilot Avionics PA 80 Video/Audio Adapter Cable (GA Twin Plug)
The Kodak Zi8 has been replaced with the Zi12 which also has the 3.5mm mic input.
All in all, if you wanted a cheaper option, I think this is the way to do it. The Zi8 and Zi12 both have the standard mounting threads so you can buy a mounting kit for it if you wanted.
I've taken some good videos, and with the audio input, it means ínstead of endless noise you can at least you can record the conversations onboard.
All in all, I think this setup cost me about 160 pounds.