I've been trying to set up the feature as described here: http://help.machform.com/m/using-machform/l/559702-creating-payment-form-with-quantity-calculation but couldn't get it to work following these instructions.
After a while of Googling and generally fumbling around, I discovered there's a fundamental link/step missing from the documentation: you need to go in to the Payment settings for the form, enable the payment gateway, select "variable amount" and link up with the choice field you've created on the form. Here you also specify the price per option.
Perhaps I missed those instructions from somewhere else (?), but it would be useful to have the page I linked to above updated to show these mandatory steps, as without them this simply doesn't work.