Basically you have 2 tools you can use:
1. Use custom logic in you controller on landing page. If querystring = yadda yadda then add this text to viewmodel. You probably want a separate viewmodel as well then so you can add a few new properties like discount above or similar. See alloy site for example how to use a custom viewmodel.
2. If you also want different content that the editor can change for different querystring values, you can either add extra properties on the content type like Discount1 Discount2 etc and use logic in controller to determine what to show to use OR use visitor groups. With visitor groups you can set up rules to let episerver determine what content to use.
Visitor groups give power to editors to create logic themselves. If you don't need that then it's usually easier to write a few lines in controller that checks querystring and outputs something based on that to viewmodel and view...