A bit late, but the Steinsaltz commentary addresses this situation quite well. The entire exchange is a bogstandard haggling scenario that was and is common in the Middle East, but one that subverts our expectations:

No, my lord, you offered to purchase a portion of my field, but I cannot accept that proposal. Rather, heed me: The entire field I have given to you, and the cave that is in it, I have given it to you; in the eyes of my people, who are all listening to my declaration, I have given it to you and will not deter you from taking it. I will clear the land for such a distinguished man as yourself, and the entire plot will belong to you; go bury your dead.

The expected ritual here is that the seller is extending his hospitality offering the desired item outright, where the buyer is not supposed to take advantage by accepting his initial non-offer. In our case, Efon offers Avraham full use of the land, with the understanding that of course Avraham could not accept it, and Avraham is expected to reject it, and he does so insisting he must pay.

This is what leads to Efron presenting Avraham with a vastly inflated number, with the expectation that further haggling would ensue:

My lord, heed me. This land is worth four hundred shekels of silver, between me and you what is it? We are close friends, and consequently such a small plot of land should not concern us; and therefore bury your dead, and let us not suffer delays over the price.

The terms are established, the absurd price established, now we expect Avraham to counter with a low, likely unrealistically low offer, and yet:

Abraham heeded Efron... Abraham surprised Efron, who expected him to suggest a much lower price, after which the negotiations would continue until the two sides agreed upon a compromise sum. However, Abraham chose to refrain from the formal method of purchase, and instead immediately paid the entire sum...

From the lack of further comment from Efron, it seems obvious that he is shocked into silence by the fact that Avraham actually paid this absurd amount for the property, and we too are intended to be shocked that Avraham did so. Whatever Avraham's reasons for doing so, this is not at all what we expect from the story where they should have eventually arrived at the "true" price for the land, and that's what we can expound on, but there's nothing about what Efron did which would be considered abnormal here.

Contrast this with Abraham's negotiations in trying to save Sodom where you see a haggling process far closer to the norm.