It depends on the age of the structure, square footage, and condition.

The key features are the most important (square footage, age, condition whether it has a basement/garage, etc.)

Normally with distressed sellers there is deferred maintenance which can be expensive.I'm not sure what the square footage is, but new floors and carpet alone for an entire house could easily be 11k, so the price actually isn't all that attractive. 

I am adding value to my primary residence by adding a shower to a half bath and adding interior square footage by insulating the laundry room portion of the garage.

So because the other unoccupied unit has the same number of bedrooms owner occupied might be a no-go, but the SFH (2b, 1ba, w/garage) unit has a larger square footage and garage access, so wondering if we might be able to get use that as a means to occupy that unit.

The #1 driver of the replacement cost (and therefore dwelling coverage) is the square footage. 

With [number of bedrooms] bedrooms and [number of bathrooms] bathrooms, this home offers ample space for you and your family.Property Overview:Location: [City, State]Monthly Rent: [Enter the desired monthly rent]Available From: [Enter the date when the property will be available for rent]Key Features:Bedrooms: [Enter the number of bedrooms]Bathrooms: [Enter the number of bathrooms]Square Footage: [Enter the total square footage of the property]Parking: [Enter the availability of parking spaces (e.g., garage, driveway, or street parking)]Laundry: [Enter the laundry facilities available (e.g., in-unit washer and dryer or shared laundry room)]Appliances: [Specify the appliances included (e.g., refrigerator, stove, dishwasher, microwave)]Utilities: [Indicate whether utilities are included or not]Pet-Friendly: [Specify whether pets are allowed or not]Outdoor Space: [Describe any outdoor amenities like a backyard, patio, or garden]Additional Features: [Mention any additional features like hardwood floors, fireplace, or walk-in closets]Property Highlights:Spacious and well-lit interior with [describe any notable architectural details or design elements]Open concept living and dining area, ideal for entertaining guestsFully equipped kitchen with modern appliances and ample cabinet spaceRelaxing master suite with a private bathroom and [mention any unique features][Add any other notable highlights or selling points of the property]Location and Neighborhood:Situated in the heart of [City, State], our home is conveniently located near [mention nearby amenities such as schools, parks, shopping centers, and public transportation options].

Alternatively, a fixed monthly fee based on estimated usage or dividing the bill by square footage occupied could simplify matters, though fairness might vary.

So to answer your other question,  the estimated cost for this build 3 times the cost of that duplex build for roughly equal square footage and including the lots and utilities and everything for both. 

Also, it may priced a little high for the square footage.