Why was Joseph sold for only 20 pieces of silver?

The Scripture plainly states that Joseph was sold to Midianites for 20 shekels of silver (Genesis 37:28). But the question is why and the Scripture does not directly say. However, the reader may infer that 20 shekels was the going rate at that time but that does not reconcile with later Israelite law for 30 shekels found in Exodus 21:32. The reader may consider the Midianites (term used interchangeably with Ishmaelites as both were sons of Abraham and wandering Arab traders) coming upon this scene sensed foul play and bargained for the lesser price for their silence. Conjecture but plausible with the ways of men, even today.

In any event, the reader can see how commentaries and the above explaination are only the views of someone and not direct from Scripture which is silent on this topic - yet provides enough information for these differing viewpoints.