What I would recommend is to edit the view files where you want to add that display to add HTML and some javascript to update the HTML to display the total amount.
It should only be a few lines of HTML/javascript.
I have added the necessary JS and HTML to show and update the total amount of products. Also with JS, I set the value of a new custom field of the item table called "total" so I can have a way to save it into the database. This field is hidden with CSS in the product page.
This is how it looks now when you go to the checkout page.
(Image01)
On the checkout, you don't even need the javascript, only the HTML since the view has an update button to refresh it when you change the quantity field.
Based on I did with the "total" custom field. How can I save the new updated value into said custom field?