Ah, managed expectations!
Sponsored search (Google AdWords, Yahoo! Search Marketing, Bing) and Web site content management are extremely interdependent.
Do you have any control what-so-ever of their website? Since the two are so intricately linked, I would be inclined to not offer an aggressive guarantee if you don’t have control of that content. Whatever you do, try to get a reasonable timeframe contracted, not just a “set-up and dump” approach, which they may be looking for.
You may or may not have a huge spike that slowly levels off of both impressions and CTs, so any guarantee should take this into account. End of the day, guarantee that you will bring them more traffic (and meaningful traffic), better support of their site, and trackable customer metrics, all at a very reasonable price.
