There are numerous factors that determine the cost of advertising on TV.  Here is a brief outline of the main variants:-

• Transmission time
  Otherwise known as ‘dayparts’, the time of day you advertise effects the price you pay as audience profile and size changes throughout the clock. Peak is the most sought-after time (1730-2300) as more people watch.  Day time and late night have less demand for the reverse reason.  However, certain dayparts are better suited to certain types of campaigns.  Post-pub is a great time to catch young men whereas daytime works best for direct response (when people are more alert and willing to act.
• Programming
  Programmes that command large, or high profile audiences are generally more expensive to advertise in that those that have smaller, more downmarket viewers.  Programmes that are in high demand are often given ‘special’ status and a premium is charged to advertisers wishing to utilize them.  Examples of specials might be films, sporting events and high rating dramas such as Lost.
• Time of year
  The season can also effect viewing levels, for example, more people watch in winter when there’s less appetite to spend time outside.  Traditionally, autumn proves expensive (in the run-up to Christmas when demand is high) whilst July and August cost less (as people holiday and spend more time outside.)
• Commercial time length
  The length of an ad also effects the price.  A 60” ad will cost twice as much as a 30” (the most popular timelength) which will cost around twice as much as a 10”.
• Target audience
  Audiences that are in plentiful supply through a channel’s schedule are generally cheaper than those that are harder to access.  For example, it would be much cheaper to buy an All Adults schedule than one aimed at women between the ages of 16 and 34.
• Regionality
  The terrestrial broadcasters provide the option of advertising regionally (to a greater or lesser degree depending on the station) or nationally.  Obviously, a regional campaign will not hit as many people as one that runs across the whole country and this is reflected in the cost.  However, some regions are more expensive than others.  For example, to hit 100,000 25-44 year olds in London would cost more than reaching the same number in Scotland as generally, London viewers consume less TV.