Child Variants x Custom size
Shopper selects variant and enters custom width.
Selectable is 100 (100$), 200 (200$), 300 (300$). Custom input is restricted to minimum of 0.1.
We assume for example 100 x 1 does cost 100$, 200 x 1 costs 200, 200 x 2 costs 400€ continues linear.
Create parent product add customfield generic child variant and customsize.
Preset according to manual and requirements. See images of the child products.
The trick is to derive the settings of customfields first, then override the values for customfield price in the childs according to the child size. See marked yellow in product images.
In this case the customsize customfield of the parent has been removed in the last step (optional).