Install central a/c in rental?

[Donald M.]

As a ballpark, how much would I be able to charge more per month in a unit in the Boston area with central a/c? I am thinking of having a mini-split system installed in the attic of a rental, with the condenser on the roof. It is a 4-bedroom, 2 bathroom but is zoned for 3 persons now. 

The unit gets a bit cold in the winter and I would like to install more insulation as well, but if I was going to have A/C put in I should do that first.