What is a fair fee for a tenant change?

[Kraig Kujawa]

Hey landlords, could you give me some input here.

I have a unit where a dad and son rent. The rent is 1050. The dad wants to leave the lease, but continue paying, but wants his son's friend to live there. 

I plan on telling him I am ok with the switch, provided the new tenant is added to the lease, the new tenant passes all the usual screens. That means tearing up the new lease, and drawing up a new one. 

But of course, this all costs me time to do. What do you all think would be a fair fee for something like this? What do you all charge?

Thanks

Kraig