Depending on the network, you may also need to pay setup costs. This covers the cost of integrating you to the network and testing tracking etc. Start-up fees can range from nothing to over $5,000. I have been asked for $30,000 before and, obviously, I never started with that network. This cost is often negotiable when you discuss contracts with the network, but it is worth noting when considering the start-up costs.
The quote above comes from a Tech in Asia article titled Twitter Story and Tips from @AskAaronLee. In the article, Aaron states that monitoring both your own brand and competitors online could provide insight on how to position your product in the future. He also says that businesses should never try to sell on Twitter, instead, they should focus on engaging and assisting their followers with relevant problems.