I just closed off a carport to expand the kitchen and to add dining room in the investment house. the return is about 350%. (i didn't use a contractor though). Of course the return on closing off carport for a garage is lower. But its not less then 40- 50%
Keller Williams Realty Atlanta Perimeter
As Lee pointed out, there are guides out there but the bottom line is that you make improvements to enhance your lifestyle then to enhance value and appeal. Sometimes the best ROI comes from an obscure or unexpected improvement - but if it matters to you then it's worth it. Of course the real world doesn't care about you so you do have to keep an eye on market ROI.
Do your homework but I would say that from both an appraisal and market standpoint, garages are always a plus.
Start with their average scale for a garage addition:
"Construct a 26-by-26-foot freestanding two-car garage, including footings and slab-on-grade foundation, 2x4 wood frame with OSB structural sheathing, and gable truss roof at 6/12 pitch. Install 25-year asphalt shingle roofing with galvanized metal flashing; vinyl siding and trim. Install five double-hung 30-by-48-inch vinyl windows; one 3-0/6-8 exterior door with half-glass and lockset; and two composite 9-by-8-foot overhead doors with motorized openers. Include 100-amp breaker at main house panel and 50 linear feet of trench buried conduit to feed new electrical subpanel. Provide electrical wiring for openers; three-way switching for fluorescent ceiling fixtures over each bay; three-way switching for two exterior spotlights; outlets to code. Interior wall, floors, and ceilings remain unfinished."
These are their numbers for our region of the country:
$57,824 cost $33,089 value 57.2% cost recouped at sale
I'm just around the corner in Decatur if you ever need more of this esoteric, copy/paste knowhow ...
Without a doubt, close it in and make it a garage with an operable door. Assuming it can fit two cars, you would probably spend $1200 on the Garage Door and Operator and another $2,000 on framing it in and putting an finish (assuming brick) on the exterior.
A garage is much more desirable and will be easier to market and sell if and when the time comes. You will thank me in January.