The price may be good, but it may end up being more expensive than if you had spent a bit more and placed your ad in the proper place at the proper time. Remember the purpose of advertising. . You may also think that it's less expensive to have the medium's staff design your advertisement. In some cases, that may be true. However, the design of your advertisement is also instrumental to the cost effectiveness.
Advertising is based upon psychology.
Many studies have been done by advertising professionals which have indicated what motivates the public. Also, the staff of a medium doesn't necessarily have your best interests at heart. Often, producing ads becomes very mechanical and attention to detail suffers. This is why so many ads contain mistakes. Also, most ad designers with the media do not have the education and training about advertising strategems and psychological studies which an agency does have.