The Canon ZR800, 900 and 930 are the least expensive camcorders with an external mic jack.
Be advised that this is only part of the equation if "extreme" audio (WAY loud or really soft) is expected.
When very low volume happens, any camcorder - in automatic audio gain mode - will "listen" for audio. This results in a characteristic "whooshing" sound.
When very loud audio happens, the automatic mic gain circuit cannot keep up. This results in a characteristic "muddy" sound because of audio "clipping" Marching bands, any amplified music or even a loud singer can cause this.
The best available external mic alone will not resolve these "extreme" issues.
Manual audio control - when used correctly - will resolve these issues. The least expensive camcorders - of which I am aware - which have a mic in jack AND manual audio control are the Canon HV20, HV30, Sony HDR-HC7 and HC9... and when manual audio control is used properly, even the built-in mics of these camcorders can do an OK job of getting loud audio (no clipping or muddy sound). But I regret that they are outside your stated budget...
I'm researching a Camcorder for my sister and am crosseyed with all the choices and info! From what I can tell, MiniDV is the way to go. She has a MacBook with iMovie08 so she can edit there and burn to DVD.
She's looking at a budget of 300-500 though $350ish would be good if possible.
From what I've read, Sony, Canon, Panasonic are good choices. I see JVC out there too. I've read a mic jack is good to have. And it needs a Firewire (iLink) jack on the cam.
So, any help on some specific models to look up and compare? She's just looking at Standard right now (no HD).
Again, mainly to shoot vacations and home movies, and she's not a techie or nerd so simple is good.