I wish the answer to your question was a simple as the question itself. Much depends upon the condition of each stamp. Check out the stamps in a catalog. It will give you a "relative" value letting you figure out which items may be more valuable than others. However, market value is a function not only of catalog value but condition and then there is market scarcity factors.
Is your collecting interest based on accumulating value (price over cost) or the ascetic pleasure of gathering stamps you appreciate or?