How does one understand the word yakar in Psalms 116:15? Classic Jewish commentators explain the word in context to mean "difficult". However, that is not how is it commonly used nor is it in consonance with how the Talmud (Chagigah 14b) understand it.
"yakar" is a word for which there isn't a good single-word English translation in this context. "yakar" is "dear" in the sense that "dear" is used in the UK to mean something costly or high priced. "Costly" in OT Hebrew as in English also has overtones of "regret" as in expressions such as "we paid a high price for that victory", or, "you paid too much for that purse". This meaning is also close to the meaning of "yakar" in modern Hebrew. The word is anthropomorphic in Psalms 116:15 and represents God as regretting the loss of his precious "hasidim", his righteous adherents in the current world. It is in this sense that the verse is applied to the tragic death of Rabbi ben Azai in the Babylonian Talmud tractate Chagiga.
[I first encountered this explanation of "yakar" in Psalms 116:15 in the introduction to Ha-Siddur Hashalem Daily Prayer Book by Rabbi Philip Birnbaum.]
The background of the OP question:
The Babylonian Talmud tractate Chagigah (the holiday offering) page 14, side B deals with study of mystical concepts, referred to as "maaseh mercava", or matters of the chariot, from the chariot passages of Ezekiel. The Essenes, who Prof. Rachel Elior identifies as Tsadokite (the "Sadducees of the NT) priests who were deposed by the Hasmoneans, dealt extensively in mystical speculations based on images of the chariots of Ezekiel. Because the Tsadokites were seen as heretical, and for other reasons as well, the Pharisaic Rabbis of the Talmud were suspicious of mysticism.
The particular passage referenced in the OP is the famous story of "the four who entered the orchard", or "pardes", which is sometimes rendered in English as "Paradise". Entering the orchard or "pardes" is a metaphor here for entering into the world of mystical speculation. The four were: ben Azai, ben Zoma, Rabbi Akiva and "acher". Ben Azai died as a result, ben Zoma went insane, "acher" became a heretic, and only Rabbi Akiva emerged from the pardes of mystical study with his faculties, faith and health intact.