halacha - Maaser Sheni Coin Value

I've been reading about redeeming maaser sheni and I've noticed people mentioning redeeming maaser on a coin of value significantly less than the value of the maaser:

"For example, a cRc company which purchased 30,000 pounds of Israeli watermelon seeds had 2,700 pounds of ma’aser sheini seeds, which is worth tens of thousands of perutos. They were able to transfer that kedushah onto an American nickel or dime and had no need for a perutah chamurah." (http://www.crcweb.org/Sappirim/Sappirim%2025%20%28Jul%202012%29.pdf)

"A coin is designated for the redemption of Maaser Sheini. This coin must be worth at least a shoveh pruta. At the time of writing, a nickel is sufficient." [There is no mention of a maximum amount to put on the coin, nor of higher value coins being needed for more maaser.] (How can one eat non-certified kosher Israeli produce?)

And yet there is also mention of "filling up" coins used to redeem maaser: "Once the coin has some ma’aser sheini on it additional kedushah can be added up to the full value of the coin."(http://www.crcweb.org/Sappirim/Sappirim%2025%20%28Jul%202012%29.pdf)

So my question is: does the value of the coin used to redeem maaser sheni have to correlate with the value of the maaser, and if so how? And if not, why?

Answer

The Gemara (Erkhin 29a) records:

הקדש שוה מנה שחיללו על שוה פרוטה מחולל אימר דאמר שמואל שחיללו לכתחלה מי אמר הני מילי בזמן שבית המקדש קיים דאיכא פסידא אבל בזמן הזה אפילו לכתחלה
[Shmuel said:] "Hekdesh worth 1 Maneh which was redeemed onto a Perutah is redeemed." He said "if it was redeemed", but who said you could do that from the outset? His words were when the Temple was standing and there would be a loss of value, but nowadays even from the outset you can do that [since there's no loss to Hekdesh since no one is using the money anyway]. (my translation)

See Rambam Erkhin 8:10, Shulchan Arukh YD 294:6

So originally you would redeem your Maaser Sheni onto a number of coins equal to the market value of the produce. But now that no one is actually using the coins to buy food in Jerusalem, you can redeem any amount of Maaser Sheni onto just 1 Perutah. A coin that is worth X Perutot can be used to redeem Maaser Sheni X times. Then you can redeem all X Perutot onto 1 Perutah of a different coin. Etc. Eventually you destroy the coin.