B&H sells the optional Panasonic unidirectional mic for $150...is this the best option for that kind of money or are there similar alternatives in that price range? Also- how much of an actual improvement over the onboard mic can we actually expect?

http://www.bhphotovideo.com/bnh/controller/home?O=ProductDetail&A=showItemLargeImage&Q=&sku=276816

I've got this mic and it's not very good. I'd suggest you get something better the Me 66.

I like the SA-568

its by nrgresearch.com

Steve - you can get the Azden SGM -1X shot gun mic for $150. Not a great mic, but better than what Panasonic is offering. (It mounts perfectly in the included mic mount BTW.) And I think the one Jarred is suggesting is even less and has two pickup settings. In any event you'll also need a short XLR cable so don't forget to order that as well.

No matter what mic you get, you'll find the difference is night and day over the on board mic.