A new law is discussed. The rabbis ask if a poor person decides to pay the erech, the value of a person, there is specific law of heseg yad, the ability to pay. In an ordinary vow, a person must pay in full. Neither a priest nor the treasurer of the Temple can negate the obligation. However, in the laws of arachnid a priest can determine a person's ability to pay. That valuation must be more than one sela and less than 50 sela.
If a poor person accepts his/her value as fie sela but that is all that s/he has, Rabbi Meir says that he need not pay more than one sela. The Sages say that he must pay all five. The Mishna decides that the is no valuation of less that one sela nor more than 50 sela.
Why does the daf about the Mishna's need to include the clause "there is no valuation of less than one sela nor more than fifty sela, which seems to be a repetition? The Gemara explains that this is the establishment of the "stam Mishna", the simple and authoritative ruling of the Mishna - this is a ruling with the Sages over Rabbi Meir.
Steinsalz reminds us in our notes that the sela referred to is the biblical shekel, which was silver that weighed 20 gera. Rambam claimed the weight at 384 grains of barley or 17 grams of pure silver.