Your Chrome and Mirror Gold are optional attributes

They have a value but only if selected

Somewhere you need a price to base the 50% on

Either the price is based off the Products Price or the Price of the Selected Attributes.

So in order to do this kind of pricing, something has to make up a price to take 50% of.

Let's say the prices are as follows:


Size
Small prefix blank price 10.00 set as default
Medium prefix blank 20.00

Color
Red no pricing set as default
Green no pricing
Chrome prefix + price factor .5
Mirror prefix + price factor 2

So this would read as:

Starting at: $10.00

Size: Small $10.00
Medium $20.00

Color: Red
Green
Chrome +$5.00 (which is $10 * .5)
Mirror +$20.00 (which is $10 * 2.0)