Is Maaser Rishon Calculated Based on Quality or Quantity

Question

Regarding the law of tithing produce, that after giving Terumah the produce is still Tevel until a tenth of the remainder is given to the Levi.

If someone had 100 stalks remaining after giving Terumah, each valued at $1, he would give 10 stalks to the Levi, which had the value of $10.

What about if someone had 75 stalks left after taking Terumah, 25 stalks worth $2 each and 50 stalks worth $1 each.

a) Can he give for Maaser Rishon 7.5 stalks, regarding of their value, and potentially only give the Levi the value of $7.5.

b) Must he give 2.5 stalks of the $2 batch, and 5 stalks of the $1 batch, valued together at $10

c) If b is correct, can he give 10 $1 stalks, which has the value of $10?

Answer 1

The Mishna (Terumot 4:6) writes:

המונה משובח והמודד משובח ממנו והשוקל משובח משלשתן.‏
One who counts is [more] praiseworthy [than one who estimates]; one who measures is more praiseworthy than one who counts; one who weighs is more praiseworthy than all three of them.

It seems the quantity of the portion is dependent of volume/mass/size.

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The Mishna (Terumot 2:4) writes:

כל מקום שיש כהן, תורם מן היפה. וכל מקום שאין כהן, תורם מן המתקיים. רבי יהודה אומר, לעולם הוא תורם מן היפה.‏
In a place where there is a priest, one separates from the better [crops], and in a place without a priest one separates from the stable [crops]. Rabbi Yehuda says one always separates from the better [crops].

It seems the quality of the portion is, under ideal circumstances, supposed to be the highest. (Under less than ideal circumstances, such as a need for a shelf stable product or perhaps a situation where the food will be thrown out anyway due to impurity, there may be room to compromise on quality.)