Sum values from items on a multiple selector

Hey people, please help me here.

I am building a new house and I need to understand pricing for all items we need to buy. To clarify, here is an example:

Here I need the fiels "M2 from Walls"to show a sum of the size of all walls on the multiple select column.

My objective is for example:
When I add a new price for wall painting’s square meter
I can multiply this price to all walls on my multiple select field.

So, If a square meter of wallpaper is $10
And I have a room with 20 sqm and another with 30 sqm
It would give me 50sqm * $10
So the sum of that room’s cost would be $50

Makes sense? Can you guys help me please?

Thanks a lot

Take a look at the rollup field type.

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