You're using RobotStats and you don't have anything to pay: it is FREE. All I ask to you in return is only to put a link on your site to the RobotStats official homepage

You just have to copy-paste the following HTML code:

<a href="http://www.robotstats.com/" title="RobotStats analyzes GoogleBot's and other robots' visits">Robots tracking and statistics</a>

You can also use this small picture (copy and paste it to your site) :

Thank you!