One of the most common complaints we hear from advertisers for whom we create ads, is that the cost of creating the ad - in addition to buying the space for the ad to run - is too high. While we don't agree with this (obviously), one method we have found effective in working with long term clients, is to create ads that can be updated and refreshed, without requiring a whole redesign. A perfect example of this are Audience AV, for whom we have created hundreds of print, web, and trade show ads and materials.
Their print ads are created so that we can update the particular product's information without moving the entire ad around - for example, if a review comes along with new and better sounding quotes, we simply update the quotation and reviewer info, and refresh the colors used in the ad. Sometimes we'll be asked to add award logos the product has received, or update the product image. Working this way, initial ad creation costs are mostly upfront, then all subsequent updates and changes we make are small and fairly straightforward, making updating and running the ad much less expensive than expected.