Zevachim Chapter 10, Mishnah 5. The Mishnah opens with a rule: "Kol hachataos shebaTorah kodmos la'ashamos" - all types of sin-offerings in the Torah precede guilt-offerings.
The reason for the precedence:
As we have already learned, the reason is that a sin-offering has four placements of blood, whereas a guilt-offering has only two placements.
"Chutz mei'asham metzora":
The Mishnah notes an exception. The metzora, at the end of his period of tzaraas, brings three offerings: a burnt-offering, a sin-offering, and a guilt-offering. And in the comparison between his sin-offering and his guilt-offering - the guilt-offering precedes, "mipnei shehu ba al yedei hechsher" - meaning that the guilt-offering comes to qualify him to become pure.
Seemingly, it is difficult to understand the Mishnah simply, since the metzora is still called mechusar kippurim - lacking complete atonement - and therefore a remnant of tumah remains in him even after bringing the guilt-offering, until he brings his sin-offering as well.
Tosafos resolves this in two ways:
The central offering of the metzora regarding his purification process in the Torah is the guilt-offering, since some of its blood is placed on his right earlobe, his right thumb, and his right big toe. It is the central offering in transitioning him past his period of being a metzora, and therefore it earns precedence.
The 'qualification' of the guilt-offering of a metzora is not only for atonement, but also to qualify him to be pure again. Therefore, the Mishnah means that when there is a regular sin-offering (not the sin-offering of a metzora) versus the guilt-offering of a metzora, the guilt-offering precedes, because it accomplishes two things: it helps achieve atonement for the sin that caused him to become a metzora, and it also qualifies him to become pure, since the process is done with it. Since it has these two advantages, it precedes other sin-offerings.
"Kol ha'ashamos shebaTorah ba'in bnei shtayim uva'in bechesef shekalim":
There are six categories of guilt-offerings, and they all come as bnei shtayim - in their second year - and come bechesef shekalim, meaning at a minimum price of two shekalim, as the verse states. The shekel in the verse is the sela in the Mishnah, and the point is that two silver coins are required, rather than just one.
"Chutz mei'asham nazir ve'asham metzora":
Two exceptions are the guilt-offering brought by the nazir and the guilt-offering brought by the metzora, "shehen ba'in bnei shenatam" - in their first year - "ve'einan ba'in bechesef shekalim", and they do not need to be purchased with two silver coins, but rather with one silver coin, a single sela.
Explanation:
Out of the six guilt-offerings, the Torah explicitly states regarding four of them that they are brought from an ayil (ram). An ayil is a male sheep in its second year - or more precisely, a year, a month, and a day old, having already started the second month of its second year. The other two guilt-offerings are brought from a keves, which is a male sheep in its first year.
The four primary guilt-offerings - the guilt-offering for robbery (asham gezeilos), the guilt-offering for misuse of consecrated property (asham me'ilos), the guilt-offering for a designated maidservant (asham shifchah charufah), and the pending guilt-offering (asham talui) - are brought from an ayil, and are also purchased at the higher price of two shekalim derived from the verse.
The guilt-offering of a nazir and the guilt-offering of a metzora - are brought from a keves in its first year, and there is no need to pay the two coins for them.
Regarding the coins:
There is only one verse that says "with a valuation of silver shekels," and it is stated in the context of the asham me'ilos, which is purchased for two shekalim. However, there is a gezeirah shavah (verbal analogy) that connects this law from the asham me'ilos to the other three guilt-offerings, but does not connect it to the guilt-offering of a nazir or the guilt-offering of a metzora.
It turns out that according to Torah law, the guilt-offering of a nazir and the guilt-offering of a metzora do not have a fixed amount, and can even be worth a perutah. But the Rabbis instituted that ideally it should be worth one silver coin - a sela - instead of two. That is to say, the guilt-offering of the metzora and the nazir can be worth just one silver coin, in contrast to the asham me'ilos, asham gezeilos, asham shifchah charufah, and asham talui, which must be purchased with two.
In summary: In this Mishnah we learned the rule that all sin-offerings precede guilt-offerings, because of the four blood placements of a sin-offering compared to the two of a guilt-offering, and the exception to the rule regarding the guilt-offering of a metzora, which precedes the sin-offering because it is brought to render him fit (hechsher) - with two explanations in Tosafos: either because it is the central sacrifice in his purification process, or because it contains two virtues, atonement and fitness for purification. We also learned that all guilt-offerings are brought from animals in their second year and for silver shekalim, except for the guilt-offering of a nazir and the guilt-offering of a metzora, which are brought in their first year and for one silver coin. Finally, we learned the source for this: the four guilt-offerings regarding which an ayil is stated and which are learned through a gezeirah shavah from the verse "with a valuation of silver shekels" concerning the asham me'ilos, as opposed to the two guilt-offerings that are brought from a keves in its first year.