Why does it mention the wealth of the land?
Radak: Just like the land became full of silver and gold and horses, that there is no end to the storehouses and chariots, so it became full of idols (8). It says "va'Yishman Yeshurun va'Yiv'at" (Devarim 32:15), "v'Chesef Hirbeisi Lah v'Zahav Asu la'Ba'al" (Hoshe'a 2:10). This was in the days of Shlomo. Eretz Yisrael was full of silver and gold - "va'Yiten Shlomo Es ha'Kesef bi'Yrushalayim ka'Avanim" (Melachim I, 10:27), and they brought to him gold from Ofir, Malchus Sheva and all kings. There were numerous horses - "va'Yhi l'Shlomo Arba'im Elef Urvos Susim" (Melachim I, 5:6). In Shlomo's day they began to make idols and serve them.
Malbim: This magnifies the sin. It was due to not appreciating the good [that Hashem bestowed on them]. Amidst great good and success, they rebelled.
What is "Ketzeh"?
Rashi: It is an expression of Ketz (end). Radak - there is no end to the number. This is an exaggeration.
