What is the best and most affordable external mike to use while filming interviews with the Panasonic HVX200P?

Well what kind are you looking for?

There's shotgun, lavalier, handheld, boom mics...

It all depends on what type of interview you're doing. If it's a set up interview with lighting, people sitting, etc... then you can mic them with a lav.

If it's "on the street" type mic with a person interviewing (like the news)... then a handheld would be best... if it's a little more guerrilla style, then a shotgun mic.

The best mic that's still in a truly affordable range is the Senheiser MKH416, and you can find those for around $800. If you need to go lower, the Senheiser ME66 is obtainable at $350, used.

Thank you for your responses. Yes, the type of interview I am doing is in a quiet place under semi-controlled conditions, like in peoples homes, or in a studio or office that they work in. They are seated and each interview usually ends up taking about an hour.

Make sure you turn off appliances in these homes and the A/C!. you dont want to film with your new mic and get to post only to realise the sound is crap. just a little suggestion....