I have never seen why they spend money on them. Everyone knows right now how he/she/it is going to vote (like Daddy did) and nothing is going to change that.
Steve, they must make even more money on pollical ads. Supply, and demand.
Add costs/prices are based on length, time slot, popularity/ranking of the host show, frequency of add or adds. I highly doubt politicians would tolerate paying more for the same time and time slot than say Budweiser or GM.
I would imagine that the political season provides the stations with chances to sell slots to the candidates that would otherwise go unsold, resulting in the station having to fill the time with self-congratulatory worship of their own, largely useless, weather departments.