What is a Kesitah?
Rashi: A Kesitah is a Ma'ah (a small coin). 1
Targum Onkelos: A sheep. 2
Targum Yonasan: A pearl.
What is the significance of the fact that Yaakov purchased a portion of field?
Rashi (to Shemos 6:9): Although Hashem had promised him the land, he did not complain when he had to purchase a plot to pitch his tent.
Ramban: Refer to 33:18:2:2. He made a point of owning the portion of land on which he built the Mizbe'ach, since one cannot sing to Hashem and praise Him on foreign soil. 1
Moshav Zekenim: He did not buy everywhere that he pitched his tent. However, here he built a Mizbe'ach; he bought the place lest people destroy the Mizbe'ach afterwards. Other places where the Avos built Mizbechos were Hefker, i.e. desolate mountains. 2
Malbim: He showed that he intended to settle there. He paid a large amount for it!
See Tehilim 137:4.
The Pesukim connote that they built in inhabited places (Shechem, Be'er Sheva)! Ha'amek Davar (to 33:18) says that when Avraham came to Shechem,, it was not a city. Perhaps the same applies the other places. Or, they built on nearby mountains (PF).
What is "Chelkas ha'Sadeh"?
Ha'amek Davar: It is a plain. Even today it is known to be a special [flat] place of Eretz Yisrael, which is [primarily] a land of mountains and valleys.
