@orangey03 – I think that online ads are less expensive because you don't have to pay for the ink and paper that print publications use to produce ads. However, if you are going the ad trafficking route, you have to pay an ad manager for his services, instead of just buying an ad outright.
I've never used an agency that offers this, but I am assuming that a large part of the fee would be for determining the best placement of your ad and serving as a medium. I have no idea what this costs, but it is probably a good bit more than the cost of the ad itself.
However, if you are not a graphic designer, you probably will need help creating and placing your ads. The service might be worth it.