I'd like to offer business-skills seminars, as web video, and quantify their value - what are some ways to do that?
A few examples include:
The claim is that for the average student, for every year of college you complete you add 8% to the salary you earn (compared to a person who did not complete that extra year of learning). Learning a new skill can not be related to those statistics because that is not the same as a year of college and you have no studies to back you up.
If you actually want to quantify the value of learning a new skill you are going to have to do some research, looking for salaries of people who have that skill and salaries of similar people who don't have that skill. It would take a skilled researcher to find this data.
If you have no data to backup your claim it will come across as more marketing hype and eventually some government agency may decide to investigate you for false claims.