This shabbat I came to a clearer pshat understanding of the last chapter of Sefer Vayikra (27) than I had in the past, and I want to summarise it here.
The chapter covers four subjects: arachin, bechorot, herem, and maaser. In more detail, the chapter divides as follows:
1) Arachin of a person: אִישׁ כִּי יַפְלִא נֶדֶר בְּעֶרְכְּךָ נְפָשֹׁת (verses 1-8)
2) Arachin of a kosher animal: וְאִם בְּהֵמָה אֲשֶׁר יַקְרִיבוּ מִמֶּנָּה קָרְבָּן (verses 9-10)
3) Arachin of a non-kosher animal: וְאִם כָּל בְּהֵמָה טְמֵאָה (verses 11-13)
4) Arachin of a house: וְאִישׁ, כִּי יַקְדִּשׁ אֶת בֵּיתוֹ (verses 14-15)
5) Arachin of an ancestral field: וְאִם מִשְּׂדֵה אֲחֻזָּתוֹ יַקְדִּישׁ אִישׁ (verses 16-21)
6) Arachin of a non-ancestral field: וְאִם אֶת שְׂדֵה מִקְנָתוֹ, אֲשֶׁר לֹא מִשְּׂדֵה אֲחֻזָּתוֹ, יַקְדִּישׁ (verses 22-24)
7) The "holy shekel" is defined as 20 "gerah" (verse 25)
8) Bechorot, kosher and non-kosher (verses 26-27)
9) Herem: אַךְ כָּל חֵרֶם אֲשֶׁר יַחֲרִם אִישׁ לַה מִכָּל אֲשֶׁר לוֹ, מֵאָדָם וּבְהֵמָה וּמִשְּׂדֵה אֲחֻזָּתוֹ (verses 28-29)
10) Maaser of produce: וְכָל מַעְשַׂר הָאָרֶץ (verses 30-31)
11) Maaser of animals: וְכָל מַעְשַׂר בָּקָר וָצֹאן (verses 32-33)
12) Conclusion (verse 34)
In all of 1-6, the word עֶרְכְּךָ reappears over and over. This word also appears in Vayikra 5 (where the "asham" animal is described as an עֶרְכְּךָ for the person who has sinned) and in Bamidbar 18 (where the 5 shekel redemption payment for a firstborn male is described as עֶרְכְּךָ, and has the same 5 shekel value specified for baby boys in Vayikra). Besides these places, each of which is "arachin" in the human sense, the word never appears in the Torah. But here, it appears in connection with animals and land, as well as humans.
In addition to the three categories of "arachin", there are three categories of "herem" - אָדָם וּבְהֵמָה וּמִשְּׂדֵה אֲחֻזָּתוֹ - human, animal, and land! It seems that nearly any object can be sanctified in one of two ways - either "arachin" or "herem".
What is the difference between "arachin" and "herem"? It seems the difference is that "arachin" may be redeemed to a cash value, while "herem" can never be redeemed. Let us go through how this works in each case.
Of the six types of "arachin", four (humans, non-kosher animals, houses, and purchased fields) seemingly *must* be redeemed. A fifth (ancestral fields) *may* be redeemed, but only until the yovel, and if you miss this deadline, it becomes "like the herem field" (verse 21), i.e. non-redeemable! Just one type of "arachin" can never be redeemed (kosher animals) - even switching them to a different animal doesn't work. Perhaps this is not actually a type of "arachin", it is just included next to the similar law for non-kosher animals for completeness.
As for "herem" (verse 28-29), it applies to people, animals, and fields - but only to ancestral fields, not purchased fields. A purchased field was originally someone else's ancestral inheritance, and it would be unfair to me to deprive someone else of their ancestral inheritance by making it sanctified without ability to redeem.
You may wonder, where do these unredeemable goods go? To the priest. (Source: Bamidbar 18:14, and of course Vayikra 27:21)
What about verse 29, which says that human "herem" must be killed? Verse 28 specifies that the "herem" it refers to must be owned by the giver. Based on this, one would suppose that human "herem" refers to one's slaves. If so, then verse 29 seems to suggest that one may freely get their slaves killed by sanctifying them (!). This seems not only morally outrageous, but incompatible with other laws in the Torah. Should knocking out a slave's tooth lead to the slave's freedom (Shemot 21:26), but killing the slave has no punishment? A master is required to give his slave gifts when the slave goes free (Devarim 15:14); should the master be able to "save money" by killing the slave a day before his release? These possibilities are absurd. Therefore, it seems impossible to understand verse 29 as referring to a slave. The commentator Shadal notes that the "herem" in verse 28 is a gift "to Hashem", while in verse 29 this is not mentioned, implying that it is a different kind of "herem". Rashbam says "herem" in this verse refers to a death penalty convict who may not be redeemed; Ramban suggests it refers to the enemy population of a city that Israel has sworn to conquer (as in Bamidbar 21:2).