What you also have to remember is that your user inputs on your page will be of text type string.
https://support.wix.com/en/article/about-user-input-elements
With the choice of these only: Text, Password, Email, URL or Phone Number.
https://support.wix.com/en/article/adding-and-setting-up-a-text-input
Whereas you have most likely put your dataset fields that will contain your prices or whatever number values you have chosen, as the field type of number.
So there is one issue straight away, the user inputs are working in the text string whilst the dataset fields are working in the number value.
To change a number value into a text string, then you need to use toString.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString
https://www.w3schools.com/jsref/jsref_tostring_number.asp
To change a text string into a number value, then you need to use parseInt.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt
https://www.w3schools.com/jsref/jsref_parseint.asp
That is why, if you look at the first example, you will see that the user inputs are designated with .text and not .value and String is added to the actual calculation.