Cart Variant + Calculation Rules

[jjk]

Any chance to apply calculation rules to a simple cart variant? Yesterday I was experimenting with various parent/child and product variant settings, but didn't come up with a satisfactory solution.
I have many products where a single variation determines the price. Product description and images are identical, so it doesn't make sense to add i.e. up to ten child products. Basically I just need a custom field defined as a cart variant dropdown list (would be ideal). Problem with this is that I can't enter the cost price (varies for each cart variant) and apply the same calculation rules to it as for the basic product.
Any hints on how to achieve this are welcome...

[Milbo]

atm only the baseprice modification is working, but we want to add a fixed costprice for it also.