Khums

As-salamu 3alaykum wa rahmatullahi wa barakatuh,

How should be paid the khums and how much should be paid precisely? 

BarakAllahu fik

wa alaikum salam wa rahmatullahi wa barakatuh,

brother the imam rassi society has translated the beneficial summary on the basics of fiqh, there you can find this:

The Fifth (Al-Khums)
It is due regarding three:

Game from the land and sea, what is extracted from them both, and what is taken from their surfaces; even if it be wood, herbs, or honey that is not owned; War booty; Taxes, sales, and what is taken from the protected caste.

It is distributed to those mentioned in the verse: {And know that from the war booty that you obtain, a fifth is for Allah, the Messenger, the relatives, the orphans, the needy, and the wayfarers—if you believe in Allah and what He revealed to His Slave...} (Q. 8:41).

The mandate of its distribution is to the imam.

wa salam

Where can I find this book brother?

What is counted as "sales"? Would income/wage count as such?

you can find it here:  the beneficial summary

i dont know if a modern wage style would be counted as sale because the shariah is very precise in its wording and ruling
so when the scholars said/wrote sales i dont think that wages are included in this.

Even if that were the case that doesnt change the fact that only the righful imam can distribute it.

wa salam

So there is basically no more zakat and khums in the zaydi school since there is no imam at the moment?

salam!