How to have a different price impact on reduced price than on sale price?

[colorsofwind]

one product may have many different attributes, so here comes the "product combinations".

But I found, in 'combination' tab, I can only set one price impact, for all prices of this specific product.

 

For example:

 

A product price like this:

$30 $50

1st attribute price like this because it has a increase impact of $10:

$40 $60

 

But I want the 1st attribute price like this:

$40 $50

So actually the impact is $10 for sale price but $20 for original price.

 

 

How can I do that? I think I should modify some files, but don't know what to modify.

Thanks a lot if you can help!