I faced this problem in my implementations, when you define a product with a set of attributes, then you find Odoo deals with all it variants with one cost price in contary to the way it deals with variants in the sales side (using "Price Extra" for each attribute).

E.g. if you have product A have attributes {colors (white, red, blue), size (small, Medium, Large), and type(X, Y, Z)} then you have 3*3*3=27 variants. But not all these variants have the same cost price (the larger the size, the higher the price).
Hope I explained well.

Any Ideas about this problem?