Spikenard

What is "spikenard" (John 12:3)?
Spikenard is a plant that grows only in the Himalaya mountains of India and Nepal. Mary's fragrant oil of spikenard was a rare, imported product in Israel.

How much volume was "a litra of a very costly oil of spikenard" (John 12:3)?
λιτραν (litran), the original Greek word of Latin origin that is translated "a litra," means a Roman pound, which is equivalent to 329 grams (11.6 ounces), which is just a bit smaller/lighter than a can of Coca Cola. 

How costly was this litra of spikenard Mary used to anoint Jesus' feet?
One denarius was the wage for one day's labor, so if Judas' estimate of the value of Mary's spikenard is correct, "three hundred denarii" (John 12:5 below) would have been someone's annual salary, equivalent to several tens of thousands of dollars today. The litra of spikenard Mary used was indeed "very costly" (John 12:3).

According to Jesus, why did Mary use spikenard to anoint His feet?
To prepare His body for "the day of My burial" (John 12:7).

If she’s preparing His body for burial, why anoint just His feet?
The passage above doesn't preclude her from having anointed other parts of His body, and the account in Mark indicates that she poured it on His head as well: "And being in Bethany at the house of Simon the leper, as He sat at the table, a woman came having an alabaster flask of very costly oil of spikenard. Then she broke the flask and poured it on His head. But there were some who were indignant among themselves, and said, “Why was this fragrant oil wasted? For it might have been sold for more than three hundred denarii and given to the poor.” And they criticized her sharply. But Jesus said, “Let her alone. Why do you trouble her? She has done a good work for Me. For you have the poor with you always, and whenever you wish you may do them good; but Me you do not have always. She has done what she could. She has come beforehand to anoint My body for burial." (Mark 14:3-8)

What discrepancies seem to exist between the accounts in Mark and John?
Mark says the supper was "at the house of Simon the leper," not Lazarus, and that "they," not just Judas, criticized Mary.

Does that mean there are errors in the Bible?
The account in John doesn't say the supper was at Lazarus' house. The "they" in John 12:2's "they made Him a supper" refers to the inhabitants of Bethany who most likely wanted to honor Jesus for the mighty miracle He had performed during His previous visit. Moreover, the facts that "Martha served" and "Lazarus was one of those who sat at the table" (John 12:2) would have been foregone conclusions that wouldn't have been mentioned if the supper had been at their house. As for criticizing Mary, Judas started it, but given the value of what she used, the others had joined in until Jesus put a stop to it. Whenever Biblical details seem to conflict, they turn out to be cumulative, not contradictory. There are no errors in the Bible.