How much compensation does an unruly passenger have to pay to an airline if the flight has to be diverted to remove them? If a passenger becomes unruly and the captain decides to land and remove that passenger from the flight, how much does the passenger have to pay the airline for the cost? 

In addition, what are the reasons for flight crews to decide whether to divert or continue the flight?
Unruly passengers can face the following:


Monetary fines
Prison sentences
A ban from that airline


Excerpt from Wikipedia:


  In the United States, passengers who disrupt the duties of a flight
  crew member can face fines up to $25,000 and sometimes lengthy prison
  sentences.


IATA has a policy for these passengers mentioned here, and more information is here.

ICAO has a detailed document on unruly passenger prevention and management. They have adopted a new protocol to amend the Tokyo Convention.