Not too long ago, the website internships.com ran a marketing campaign on Twitter, appointing Charlie Sheen to advertise for an intern on his Twitter page. Sheen currently has over 3.5 million followers. The campaign supposedly cost at least $100,000 and included only one post by the Two and a Half Men star:
As a result of the campaign, internships.com experienced 1,035,021 unique visitors to their site, generating a low cost-per-click (9 cents). What was probably even more thrilling to the company was that over 80,000 people actually filled in an application! This would result in a CPA (cost-per-action) of $1.21, which is to be considered as extremely low.
This example goes to show that paying a celebrity to tweet about your company can be highly cost-effective. Just make sure the star has got a big number of followers and that the fanbase incorporates the potential customers you want to target.
Apparently it doesn't matter if the celebrity you choose for your ad is sane or not...