A price cascade offers a discount membership to surfers who closed your join form without joining. You can make this offer from both the pre-join and join forms.
Go to the Programs Admin page and start a new Program. Name the program, and give it a short name. Select standard as the type and set the payout of 'default'. Check the Program is Hidden box. This prevents affiliates from sending traffic directly to the discounted join links, but still gives them credit for the sale.
Go to the Sites Admin, edit a site, and add the discounted price in the Site Options section. On the main sites page, click the Special Payouts icon and disable that pricing option from everything except the program you previously created.
Now go to the Popup & Exit Admin and add an exit chain that swaps the current program with the program you previously created. The Popup Chain will just contain that one site.
Create a file to redirect the surfer to the following URL:
Replace linkdomain with your linkdomain. Replace X with the ID number of the program you created earlier. Add the URL to this file in the URL box of the Popup/Exit admin. Don't add a rule for this new chain so it never gets automatically picked. Add this chain to the site's join page so it will pop if the surfer closes the window.