First you need to set up a payment form. You can refer to the link below.

http://www.dnnsmart.net/Support/Articles/tabid/222/ContentId/9/language/en-US/How-to-set-up-a-payment-form.aspx

 

The subtotal function only displays the prices of certain fields in groups. Here is a screenshot of the setup.

Here are the setup steps.

1. First edit the fields in the step wizard. Subtotal is also a field type. You need to enter a formula to calculate the result.

2. Then set a total amount on the module Settings > calculate & payment options page.

3.The form has been set up.

 

You can download our sample form in the link below. You can use this example form to quickly understand how to set up.
http://dnnsmart.net/portals/0/articleimages/subtotal_example.xml