Chair Offers COVID-19 Compensation... But Asks ME to Put Out a Number
There are really two parts to the Chair's question:


How much additional work have you had to do so far? What additional work do you anticipate going forward? Are there any time savings that should also go into the calculation?
What is the appropriate renumeration for that work?


You should be able to estimate the answer to (1), and explain where the number comes from. You should certainly include this information in your response to the chair. Then, as in the answer by @Allure, you can propose that they pay you for the additional time at (broadly) the same rate as you usually expect to get. For example,


  Hello,
  
  I estimate that I've had to spend 80 hours preparing new material, and
  there will be an additional 4 hours work each week going forward.
  However, I'm saving an hour on travelling between campuses. Assuming
  this continues for the next 12 weeks, we're looking at something like
  120 hours additional work. I think my usual salary works out at around
  $30/hour.
  
  Thanks, 
  Mndifldz


Note that all you have done here is state facts, so there is nothing anyone can criticise you for. You leave it up to the chair to do the maths, and decide what they can afford to offer. However, you lead them towards a number that (hopefully) you think is fair.