Tenant breaking lease - what's reasonable response?

[John Kennedy]

I have a relatively pricey rental ($1,950/mo) with a tenant who renewed their lease last November for an 18 month term ending in May 2022.  Then, they found a house they "fell in love with" and ended up buying it.  They contacted me in January to let me know they would be moving out March 1.   I of course reminded them of their lease obligation, and let them know they would be responsible for the lease payments until I could backfill it.  So far no takers though.  

My question is...what would you all say is a customary or reasonable termination fee if I were to agree to that rather than continuing with the "monthly rent until backfilled" approach, thereby taking on the risk and letting them move on with life.  Right now, rental activity is a bit slow due to the time of year, but I expect activity will be picking up as we head into spring. 

The tenant is acknowledging their responsibility...no problem there...but has asked if I would consider a settlement.