eretz arba me'ot shekel-kesef - A land [worth] four hundred silver shekel
The above is the translation meaning according to Onkelos.
Another meaning of these words could be that 'Efron suggested that either he or his ancestors had paid this sum of money for this piece of land. Our Sages in Bereishit Rabbah 58:7 took the view that the piece of land in question was not worth anywhere near the price 'Efron was charging Avraham. This is why his name is suddenly (v16) spelled without the letter vav, to show that instead of becoming rich by overcharging Avraham, 'Efron became impoverished as a result of his greed.
On the words "what is such [a paltry sum] between me and you," the Midrash Bava Metzia 86 says that the righteous promise little and perform in excess of what they promise. An example is Avraham who promised to serve his guest only a piece of bread, whereas he prepared a sumptuous feast (Bereishit 18:8), instructing Sarah to bake cakes from the finest flour, while he himself rant to the stables to prepare the best calves for the meal. The wicked, on the other hand, promise a lot and do not keep any of it, such as 'Efron who kept speaking about what he was going "to give" to Avraham whereas when it came to the conclusion of the transaction he overcharged him. The Torah testified that Avraham paid him in hard cash, the price being exorbitant. As proof of the miserly and avaricious attitude of 'Efron the Sages in the Talmud draw our attention to the numerical value of the name 'Efron which the Torah abbreviated to when it became clear that he did not give Avraham anything. The remaining letters in his name amount to 400, the same as the numerical value of the letters in the expression ra ayin (grudging, envious).
Avraham did not want to accept any gifts from him but to pay full value (bechesef male), as it is the custom of righteous people. We find that King David also did not want to accept any gifts and paid handsomely for the threshing ground of Aravna the Yevusi (2Shmuel 24:24), although offered the site plus animals to serve as sacrifices as well as the animals' yokes to serve as firewood. David paid 50 shekels for the threshing ground plus the animals and their yokes as he did not want to appear cheap in the eyes of G-d.