Is a roof deck on a multifamily worth building?

[Johann Jells]

I have 2 properties that could benefit from roof decks, a 3U that already has a common yard and a back patio exclusively for the ground unit, and a 4U that the ground unit has a private yard. The latter is 3 large units with a basement studio. All the units in both rent for between 1400 & 1800 excepting the studio. 

So, do I just try and figure out the payback period for the investment like any other property move? I've heard prices up to $40/ft! Both would also require extensive stair work for access. But it's not clear to me what premium I can charge in that price point. My gut is that it's a far more important feature when selling a condo than renting an apartment, but I often hear from tenant prospects how much they'd like it. If it costs $25k per deck and I can only get $150/month more for the 2 units per per property that have no deck currently, does that make financial sense? That's a GRM of 13.8. Would I even get that much more? I guess I'd have to speak to local rental agents for that. Anybody been down this road?