Rashi understands the first quote as "Between two such friends as we are, of what importance is that? Nothing at all! Leave business alone and bury your dead!"
Sforno expands, "“after all it is a very minor transaction! It is so insignificant a matter that the acquisition can be made by a mere declaration without being recorded in a document. As soon as you have handed over the money you may consider yourself as burying Sarah in your property.”"
The Radak explains it as "what difference does such a paltry sum make, whether you want to pay it or do not want to pay it, go ahead and bury your dead."
With the Rashbam echpoing "Take it and bury your dead for free if you so desire.”
The Rabbeinu Bahya has a more complex take on it as follows:
On the words ביני ובינך מה היא, “what is such a paltry sum between me and you,” the Midrash (Baba 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 guests only a piece of bread, whereas he prepared a sumptuous feast (Genesis 18,8), instructing Sarah to bake cakes from the finest flour, while he himself ran 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 עפרון which the Torah abbreviated to עפרן 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 רע עין, grudging, envious.
Avraham did not want to accept any gifts from him but to pay full value, בכסף מלא, as is the custom of righteous people. We find that King David (Samuel II chapter 24) also did not want to accept any gifts and paid handsomely for the threshing ground of Aravnah the Jebusite, although offered the site plus animals to serve as sacrifices as well the animals’ yokes to serve as fire-wood. 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 (Samuel II 24,24).
The second quote is a statement that the currency/coins that Avraham used was in a form that a merchant would accept (the highest quality, according to some commentaries). The point is that the specie used was above reproach in its value.
(all quotes from the sefaria page)
