A “Websert” (as called by DM News and others) is a third party offer added to a transactional email.  For example, the image here is of a websert added to my hosting bill.  Every month my web hosting company sends me a bill and add in a third party offer.  I’ve bought this kind of placement in the past and found that if the product / service being offered has some synergies to the email the recipient is receiving that conversions can be surprising.

In my experience, these webserts cost similar to sends emails to third party lists.  They greatly depend on the quality of the list.  Expect to pay between $10 and $100 CPM but could go up to $300+ cpm for certain lists.