Joe Skinner

Canon and Nikon are pretty similar, though Nikons tend to be a bit sharper more often than not, especially when you get to the higher end primes that are also more contrasty and colorful. There are adapters for each specific brand/lens mount. Even the cheap ones work (around $25), though may need a bit of tightening (slip a knife in the prongs of the mount). If you look hard enough, you can probably get an old F/1.8-2.8 prime between 28-55mm without issues for as low as $50, though they usually hover around $100. Go wider/longer/faster and the price starts to go up.

The biggest limitation when using primes on a GH2 is when it comes to wide angles. You'll usually find down to 28mm cheaply enough, but things start to get more expensive past that. Primes tend to bottom out at 20mm, too, which is about a 40mm equivalent on the GH2. Not very wide. I have a Nikon-mount Tokina 11-16mm F/2.8 so I can go wider than that (about 22-32, wide enough), though it's above your suggested budget at $650 (and needs a slightly more expensive adapter because no aperture ring). Sigma makes a cheaper, slower, wide angle lens that can work, too. A bonus to these is that under good lighting, you can use ETC mode's additional 2.6x crop for a normal field of view. Like having two lenses in one! :)

You talk about using a tokina 11-16. I was looking into getting a GH2/3 and was going to get this lens. But lots of adaptors say they only deal with focal lengths of above say 25mm; mind these are the cheap ones. Was just wondering what the difference between cheap adapters and expensive ones our apart from build quality and apature control. How can they use wide angle lenses when some of the cheap ones can't??? Thanks