“He spoke to Ephron in the hearing of the people of the land, saying, “If you will only please listen to me; I will give the price of the field, accept it from me that I may bury my dead there.” Then Ephron answered Abraham, saying to him, “My lord, listen to me; a piece of land worth four hundred shekels of silver, what is that between me and you? So bury your dead.” Abraham listened to Ephron; and Abraham weighed out for Ephron the silver which he had named in the hearing of the sons of Heth, four hundred shekels of silver, commercial standard.”
As Ephron just changed the deal (initially Abraham asked for the cave; Ephron slyly counter-offered with the cave together with the field as a package deal), Abraham accepts the terms of his offer and reiterates that he is willing to pay full price to acquire the property – there is no need grandstanding and pretending to offer it at no cost. Abraham has already had trouble with having his property taken from him (see Gen. 21:25) and at this point simply wishes to acquire the property in a real legal sense, deed and all.
However because of Abraham’s precarious standing (he is a resident alien with no real right to buy property), coupled with the fact that he is known to be wealthy, and is in need of a burial site fairly quickly, he has no bargaining power – and Ephron knows this. According to Rashi, four hundred shekels of silver would have been enough to buy a large estate, most likely a great deal more than the field and cave are worth.
For one, the type of silver mentioned was ‘commercial standard’, which meant it was to be weighed out (coins were not yet invented). The Talmud informs us that each commercial shekel of Abraham’s may have been worth as much as 2,500 regular pieces of silver (Bava Metzia 87a).
Second, consider that Jeremiah bought a field for just 17 shekels (Jer. 32:9) so this may give us some perspective.