Maaser Sheni, Chapter 5, Mishnah 4. The Mishnah explains the procedure for redeeming neta revai: "Keitzad podeh neta revai" - what is the process of redemption.
"Maniach es hasal al pi sheloshah":
The one redeeming sets down a basket, in preparation for filling it with the fruit he is about to pick, and he places it before three - three men who take part in the assessment. The reason for this will be explained below.
"Kamah adam rotzeh lifdos lo b'sela, al menas l'hotzi yetzi'os mibeiso":
How much fruit would a person be willing to redeem for one sela, on condition that he bear the expenses from his own home - that is, when the costs fall upon him and are paid out of his own pocket:
Tending the fruit and guarding it.
Hoeing the surrounding area and working the garden.
The picking itself.
It emerges that the question is how much he would be willing to pay for the fruit alone. He must deduct from the calculation the fact that ordinarily, when a person lays out money to buy such fruit, a certain amount of expenses is included in the price. That amount is not included in the redemption, since the redemption is only for the fruit itself, and therefore it must be subtracted.
Why three are required:
Here lies the reason that three are needed: the calculation is complex. This is not a simple question of the price of the fruit in the store by the kilogram; rather, expertise is needed to estimate how much capital must be invested in producing the fruit - expenses that come out of a person's own pocket and are not included in the redemption.
In this respect neta revai differs from Maaser Sheni: with Maaser Sheni this calculation is not made at all, and the assessment is based solely on the price at which the fruit would be sold.
"U'maniach hama'os v'omer":
He sets down the coins and says: whatever is gathered from this - everything that will now be picked - shall be redeemed with these coins, at such and such a number of baskets per sela. Whether it is five baskets per sela or six baskets per sela, it all depends on what was determined to be the value of the fruit, and again - without the expenses.
In summary: We have learned the procedure for redeeming neta revai - placing the basket before three; assessing the value of the fruit alone, deducting the expenses that a person lays out from his own home; the need for the expertise of three because of the complexity of the calculation; the difference from Maaser Sheni, where the assessment is based on the sale price alone; and finally the wording that designates the coins: whatever is gathered from this shall be redeemed with these coins, at such and such a number of baskets per sela.