What did Efron mean when he added "between me and you what is it"?
Rashi: Between good friends like us, [a field worth] four hundred silver Shekalim is like nothing. Abandon the sale [and accept it for a gift]! 1
Rashbam: He meant that the option to accept the field as a gift was still open (as if to say 'Why pay four hundred Shekalim when you can get it free?) 2
Seforno: The price is so cheap that the verbal agreement will suffice to consider it as if you had buried her in your property even though you have not yet paid for it.
Malbim: (a) The price is so little that it is not proper to discuss whether it will be a gift or a sale. (b) You said that amidst the money, the seller resolved to transfer ownership. This does not apply to a mere 400; it is like a gift! Therefore, you may bury only your Mes, but not make it a cemetery.
What was the significance of 400 Shekalim that Avraham paid for the Me'aras ha'Machpeilah?
Rashi: Refer to 23:16:1:1.
Ramban #1: Four hundred Shekalim was the price of the field that Efron or his father had paid for it, and that is the amount that Avraham now paid for it.
Ramban (citing Targum Onkelos): That was the going price.
Ramban (citing Chazal): Efron charged an exorbitant price, according to his fancy.
Divrei Eliyahu, Kol Eliyahu: It is proper to have 600,000 people at a Tzadik's funeral. Each person is an Amah by an Amah. They occupy 600,000 square Amos, which is eight Beis Korim. The Torah fixed redemption of a Hekdesh field for 50 Shekalim per Beis Kor.
There are seven mentions of burying his Mes. Not all are needed! Also, why do the first six put 'bury' before 'Mes', and the last puts 'Mes' before 'bury'?
Divrei Eliyahu, Kol Eliyahu: This hints to seven others who will be buried there [from now; Adam and Chavah were already there]. Six were Tzadikim (Sarah, Avraham, Rivkah, Yitzchak, Leah and Ya'akov), who are considered alive even in death, until just before Techiyas ha'Mesim. One (Esav's head) was a Rasha, who is considered dead even in his lifetime.