I saw some new, full versions of sorenson 4.5 on ebay a couple weeks back for about $235-250. We are still running 4.3 and I talked to one of their support guys at Macworld a couple weeks ago who said that there are some major stability improvements in 4.5 as well as substantial speed improvements, so it's probably worth upgrading.

I've bought a bunch of software off Ebay over the years and haven't yet had a problem with counterfeits or anything like that, particularly if you buy from a power seller. Other than ebay, not sure where else to find a screamin' deal. I do use froogle.com a lot, it seems to have about the best price search engine I've come across so far.