Adding washer/dryer to rental - worth it?

[Scott Kronzek]

I am about to close on a 3 unit and looking to add as much value as possible. Currently tenants have to walk a couple blocks to a laundry mat. They obviously have to pay to use them. 

Current market rent for each unit:

1st floor - 1/1 $600

2nd floor - 2/1 $900

3rd floor - 1/1 $750

How much would an in unit washer dryer add to each unit?