If you ask me, conversion rates depend more on your website than anything else. Global or nation wide averages don't mean much I think, because they don't show what's possible. The most successful stores are the ones you need to use as an example. And it is very likely that the most successful websites don't have the the highest conversion rates.
Just some common sense rules that affect your conversion rates:
- Few search engine refered visitors --> Higher conversion rates
- Lots of search engine refered visitors --> Lower conversion rates
- Lots of supporting content pages --> Lower conversion rates
There are more rules of course, but generally I see people complain about lower conversion rates while they shouldn't.
So what to expect is that when doing
SEO, conversion rates will most likely go down. The ultimate goal is of course profit. Getting the highest profit is finding the right balance between conversion rate, cost per conversion and total number of conversions.