How to multiply the cart value in customfields?

[hazael]

I am making a store that sells terraced houses
I made a function that adds another building in a series and calculates the width and length of the entire set
Each quantity (+/-) adds or subtracts next building. (in the photo, a view from above on the roofs of buildings).

in basket +3 = 3 connected dwellings as one building
in basket +4 = 4 connected dwellings as one building...

Now I want the customer to be able to buy more than one such created kit

in cart in custom fields I can only add or subtract price value. Can I create a select field with the ability to multiply the price?