The only apparent reason for us to know their monetary system is to give us a frame of reference for how large the bribe is that Zeezrom offers to Amulek (v. 22). A brief explanation will put this amount (six onties) into perspective. The gold coins were based on the senine, and the silver coins were based on the senum.
GOLD COINS
SILVER COINS
Senine=judges daily wage (v. 3)
Senum=judges daily wage (v. 3)
Seon=2 senines
Amnor=2 senums
Shum=4 senines
Ezrom=4 senums
Limnah=7 senines
Onti=7 senums
Therefore, Zeezrom's bribe of 6 onties is equal to 42 days of pay for a judge or 42 measures of barley (v. 7).
Hugh Nibley
"This was the system established by King Mosiah…They had a system which ran in sevens instead of fives and tens; or sixes and twelves, as the English [system] does; or the decimal system as we use it. It ran in sevens, and Richard Smith pointed out it was the best possible system that could be devised. It used the least coins for any necessary transaction. If you want to figure out a system that will use a minimum amount of coins and save you a lot of trouble, this is the system." (Teachings of the Book of Mormon, Lecture 48, p. 316)