The cost of the licence depends on the amount of rights you purchase.
A non-exclusive licence costs the cheapest because the author can sell the illustration to other people.
An exclusive licence costs more because the author may not sell the illustration for the term of the sold licence if it is to be used for the same purpose and in the same territory.
If, for example, an exclusive licence is sold in the CIS territory, the author can sell the illustration for use in China.
Assignment of copyright has a maximum value, but gives the buyer complete power over the illustration: in all countries and for all purposes.