What is this Pasuk referring to?
Ramban: It refers to a Bechor of a Tamei species. If one is Makdish it, there is no Kedushas Bechorah. It has the law of Hekdesh [Bedek ha'Bayis]. The owner can redeem it with an added Chomesh, and others redeem it for its value. 3
What does "bErkecha" mean here?
Rashi: It means 'according to the evaluation of the Kohen'.
What is the end of the Pasuk coming to teach us?
Rashi: It teaches us that, if the owner fails to redeem the animal from Bedek ha'Bayis (and pay the extra fifth), then the Gizbar sells it to somebody else.
Why does the Pasuk say first "u'Padah v'Erkecha" and then "v'Nimkar b'Erkecha"?
Rashi (in Succah, 40b): To teach us that Hekdesh goes out to Chulin either via the owner redeeming it and adding a fifth, or via the Gizbar selling it to someone else.
QUESTIONS ON RASHI
Rashi writes that our verse discusses being Makdish a Tamei species to Bedek ha'Bayis. Why don't we learn from Pasuk 11 "v'Im Kol Behemah Teme'ah"? We already know a Ba'al Mum from "Asher Lo Yakrivu"!
Moshav Zekenim #1: According to the opinion in Menachos (100a) that a Tahor Minchah may not be redeemed, even before Kidush in a Kli, we can say that Temei'ah teaches that only a Tamei [Minchah] may be redeemed, but not a Tahor [Minchah].
Moshav Zekenim #2: Temei'ah teaches that it has a [permanent] Mum that can never be fixed, similar to a Tamei species.
Rashi writes that our verse discusses being Makdish a Tamei species to Bedek ha'Bayis. Perhaps it refers to the law of Bechor in the previous Pasuk!
Rashi: It cannot refer to Bechor, since Tamei species are not subject to the Bechorah. Nor can it refer to the Bechor of a donkey, for it is redeemed for a Seh, which is given to the Kohen and is not Kadosh.