Convesion percentage is a tricky issue as it does vary significently across various industries and products.
These are very estimated numbers, but it's a starting point. When a company has no idea about what conversion rate to expect, and has a paid product, then 2-3% is a 'usual' estimate for products under $100.
As the product price goes up, the conversion rates go down. Also, when there are more than two steps to buy a product (i.e. download, register later. Partial free subscription, full paid subscription) the conversions rates go down as as well.
The inmeasurable number associated with trial and full versions is branding. While someone may dl your program for free, if they liked it - used it, they may recomend it to others who will buy the product, but because they didn't come through PPC - the PPC will not get credit for the conversion, but in reality, that sale was directly influenced by the PPC.
8-9% download of a product I think it a pretty healthy number. If 25% of the trial products were later registered, then you'd run around a 2% conversion rate.
Hope this helps a little.