I am not sure of the value of advertising online. At this point, I have never clicked on an advertisment and I have been using the web for years. Although I must glance at them not one has even registered with me except when I was thinking of buying some lamps from a bricks and mortar store but checking the prices online and suddenly realized that all of the ads on the sites I was going to were for lamps, which was a little creepy.
Time Magazine had an article August 2011 (which also linked to statistics) in which it stated:
Internet users rarely click on an ad to be taken away from a page — only one in a thousand do so, according to Google
Checking Conversion Rate and Satistics
I suggest that if you are considering using online adverting start with a low buget,and keep checking the conversion rate to see if this looks like something that is worth the investment. Most marketing companies, Social Media and Search Engines will give you an online account panel which will provide satistics for your ad campaign based on the criteria you submit, the traffic to your site via advertisments and most include the conversion rate.
There are a number of agencies that will work with you to place your advertisments on websites but by far the main source of advertising online is AdWords which is run by Google. Most other non-Google online advertising campaigns are based on AdWords and the same terms are used to explain the process.