Rabbi Sorotzkin compares it to the 100 kesitah that Yaakov paid for the land in Shechem on which to settle himself, his family and his flocks. IIRC, Rabbi Sorotzkin says that a kesitah was a quarter of a shekel (.25). Thus, 400 shekel was 1600 kesitah.
This means that Avraham paid 16 times what Yaakov did for just the burial cave. Of course, Efron "included" the field that the cave was in even though Avraham only wanted the cave and the field was useless for his purpose. Rav Hirsch and others say that Efron claimed that the field would no longer be of use to him once the cave was used as a burial site. Thus he insisted that the field go along with the cave.
I would use an analogy as buying an entire cemetary in order to be able to use one plot.
In any case, Rabbi Sorotzkin compares this to the area that Yaakov bought in Shechem which was large enough to support the entire family with its flocks. This means providing living space, pasture lands, and (at least) the kitchen gardens that they needed. At the very least, it would be a complete working sheep farm in modern day terms.
Thus Rabbi Sorotzkin says that Avraham paid 16 times for a single field what Yaakov did for a complete farm and pasture lands. He says this was definitely a case of price gouging.
Rabbi Sorotzkin says from the descriptive language used in the two cases, one can tell that the field was just a small field with the cave at the end. Otherwise different language to describe it would have been used
Rabbi Etshalom in 5773 suggested that Avraham actually did not pay for the field. Ephron "adopted" him as a member of his clan, who was therefore "entitled" to bury Sarah. Avraham paid the money as the price for the formal "adoption" into the clan. That is why the exchange had to be done before the council of the Bnai Cheis. Land could only be transferred to members of the clan and adoption into the clan required a formal procedure. Unfortunately, the link is currently broken so that I cannot copy the citation.