Insha'Allah you are doing well and making the most of this blessed month of Ramadan. I hate to take up your time but I just have a short/quick question about the concept of “Nisab” for purposes of paying zakah.
I am extremely confused about Nisab……what is the purpose of it, what is the monetary value of it in today's society (here in Minnesota for example), and if a person meets the nisab requirement, is the zakat applicable on all of his/her income or only the income above the nisab value.

All praise be to Allah, and may His peace and blessings be on the greatest messenger, Muhammad.
The nissab is the zakatable minimum below which one is not considered to be wealthy enough to need to give zakat.
Ibn Qudamad said in al-‘Umdah,
“No zakat on silver until it reaches two hundred dirham [approx. 595 gm of pure silver], and its zakat is five dirhams (2.5%), and there is no zakat on gold until it reaches twenty mithqal [approx.85 gm of pure gold], and its zakat is half a mithqal (2.5%).”
This is a consensus.
Now, should we use gold or silver to value our wealth? Some say use what is better for the poor, which means in our time silver since that will mean more people will pay zakat. It seems that gold is the more realistic measure of wealth and in valuation of wealth and whether it reached nisaab, we should use it.
So the equation is easy:
Price of 1 gm of pure gold X 85 = nisaab
You can figure out the price from the paper or online.
Today, the price of one gram of 24 kt gold is 26.21. multiply that by 85, and the nisaab will be $2228. If you have 3000, you must pay zakat on the entire 3000, not the 772 difference only.
Allah knows best.