It's difficult to determine whether and how overpriced was the purchase of the burial cave. We have little insight into the price of land during this time, which is compounded as well by our lack of knowledge as to the size of the burial site. Bruce Waltke writes:
Since ancient land values and the extent of the property are unknown, it is impossible to evaluate with certainty the price according to market value. On the one hand, the price may accord with market value. Sarna notes, “Three texts from Ugarit … record real estate transactions involving a purchase price of 400 shekels of silver.” On the other hand, the price seems high when compared with David’s purchase of the temple site for fifty shekels (2 Sam. 24:24).
Waltke, B. K., & Fredricks, C. J. (2001). Genesis: a commentary (p. 320). Grand Rapids, MI: Zondervan.
Beyond the purchase of the threshing floor that David purchases, the other major purchase in silver shekels recorded in the Tanakh is the purchase of the land for Samaria by Omri for 6000 shekels (1 Kings 16:24). Concerning this purchase, Matthews (NAC) notes:
Speiser considers the price exorbitant since Omri paid six thousand shekels for the entirety of Samaria (1 Kgs 16:24), but, as Speiser himself observes, the weight of the shekels often varied among centers; thus such comparisons, especially since the Omri period is much later, is speculative (Genesis, 171).
Mathews, K. A. (2005). Genesis 11:27–50:26. The New American Commentary (Vol. 1B). Nashville: Broadman & Holman Publishers.
In Genesis 33:19, Jacob likewise buys a plot of land: one for 100 pieces of silver. The weight of measure used there, however, is unknown, so comparison is again impossible. It's worth noting though that in Genesis 20:16 Abraham receives 1000 shekels of silver to cover an offense. This would place the purchase of the field in Machpelah well within his wealth.
While the narrative makes obvious that Abraham was willing to pay whatever it would take to purchase outright the burial land, implying that Efron was in a position to take advantage of him, there are unfortunately too many unknowns to make a reasonable guess as to how overpriced the land was sold for. The variable weight of the shekel, the unknown size of the plot, and the unknown price of land at the time all make any guesses uncertain.