Banner advertising can be amazingly expensive if you don't know what you're doing. (It can be amazingly expensive even if you do know what you're doing)
If you're on a budget... Try LeadBack.com. It's part of the Advertising.com network that has a “Re-Targeting” service. They can do it on a cost of $2.00 per click or $2.00 per thousand impressions with a minimum buy of only $200.00. You just need a privacy statement on your website somewhere to participate.
You can place the LeadBack script on a PPC landing page or expose all visitors by placing the script on a site-wide footer. You can likely do something in between where you display the script based on referrer. (If referrer = Google, then display script) After they LEAVE your website... they begin to see your banner ads all over the internet.
Something I’ve been wanting to try is to place the script onto other niche websites or blogs. The prospect sees your banner ad on the blog (or not) and then begins to see your banners all over the internet after they leave the site. (I might create some websites just for that specific purpose.)
Also… The ads are displayed in places that already have ads. You’re not increasing the number of ads that the prospect sees, just targeting the ad placements that the user is going to be exposed to anyway.












