Last week I did some research on conversion rates for SEO, SEM, PPC and email.  I dug around through the following sources: the DMA (Digital Marketing Association), Marketing Profs, Marketing Sherpa, LinkedIn Answers, Fireclick, Constant Contact, Mail Chimp, Lyris, Adchemy, Conversion Chronicles, Jupiter Networks, Blue Corona Interactive, Statbrain, Alexa, Quantcast, the Web Analytics Association, Microsoft’s Atlas Institute – Digital Marketing Insights, CoreMetrics and eConsultancy.
 
The general “rule of thumb” is between 2 to 8%.  This holds true for people coming to your site from various sources for various activities – whether to register (for newsletter, webinar, etc.) through to purchasing from your site.
Caveat – if you are holding a special promotion or heavily discounted sale your conversion rate should increase accordingly.
Here are a couple of charts I found: 
 
Vertical Conversion
Rate (%)
Catalog 6.1
Specialty stores 3.9
Fashion/apparel 2.2
Travel 2.1
Home and furnishing 2.0
Sport/outdoors 1.4
Electronics 1.1
All verticals 2.3
Source: Fireclick Index
View Chart Online
Fireclick issues a weekly report gathered from the data of their customers – just Google the Fireclick Index.  Good info.
 
Here is what I could find for “rule of thumb” averages for email campaigns.

User-triggered campaigns
Open rate: 27%
Avg. CTR: 9.3%
Avg. conversion rate: 2.3%

Lifecycle messaging campaigns
Open rate: 26%
Avg. CTR: 9.3%
Avg. conversion rate: 2.3

Click-stream based campaigns
Open rate: 33%
Avg. CTR: 14%
Avg. conversion rate: 3.9%

More as it develops.