A roofer might be able to tell you that they can install asphalt shingles for $150 a square as a ballpark figure.

Some thing you should know are: When is the roof covered by replacement cost vs cash value, how does color matching shingles factor into partial roof replacements (which may lead to entire roof replacements), which shingle is on your roof now and is there a class action lawsuit effecting that shingle (there might be).  

Whether permitted or not, the slab likely has little to no footing and it's not likely it includes a vapor barrier, plus the span on the 24" on center ceiling joists is likely too far, and when you sheath and shingle it will get worse...assuming there is even a world where it would be viable to insulate sufficiently and pass Title 24 etc. 

From what I can see from the road, half the roof is covered with metal roofing, and the other half is shingles.

I have 7 properties, and all but 1 of them have the original roofs which are currently 3-tab shingles and they range in age from 11 years old to 18 years old.

New roof with sheathing (old tin-style shingles with purlins), new water heater, new electric panel with a lot of wiring, new kitchen & bathroom, and yet I still have ancient windows and old plumbing (on the plus side, someone before me had the foundation redone).

Do you really need all that shingle, or could you replace it with siding on the walls?

Conditions:  Broken/cracked driveways or sidewalks, overhanging trees, stairs without railings, pealing roof shingles, rotted wood on trim, etc. all can be issues in an Insurance company inspection

If you can’t look for potential red flags, lots of cracking in walls and ceilings, doors that won’t shut, crumbling foundations, or large cracks in foundations, updated wiring, roofs that are curling or missing shingles and will need to be replaced.

Is the roof leaking, or have missing shingles?