Custom duty/import duty query

Hi,

My import duty query is as follows: 

1) On what value is the customs duty calculated? Is it the invoice value or the market value of the product? What are the elements included in the finale value on which the duty is calculated? 

2) If the container has the main product with other free product(Consumer offer.Printed on the pack. Not free for me) what value shall be considered to calculate the duty?

3) What part of the duty can be claimed?

Answer to this or a point in the right direction much appreciated.

thanks