Display Product Cost+ User Based input

I am trying to create a custom block where a user can add his own cost on product cost. Which means, product cost+ customer cost, and display total price. But only displaying not adding that total cost in final amount.
Ex- product price is 100 rs and customer adds 100 more. then calculator displays 200 rs. but it doesnt sums it up in final amount. Customer still has to pay only 100rs.
here it is the image u will surely get the idea.
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• This topic was modified 6 years, 3 months ago by kshitiz621.
• This topic was modified 6 years, 3 months ago by Steven Stern (sterndata).