Great question. I would start with understanding/break down the parts of a home and evaluate how trade workers charge for the job.
____Repair cost break down___
Roof - repair price depends on home SQFT, type of roof + any know damage
HVAC - replacing a Condenser (outside unit), handler (inside unit), ducts or possibly replacing all together
Electric - cost depends on the job - replacing panel, running new wires, adding or repairing electric to an area
Hot water tank - cost of tank + labor + ( some plumbing components)
Windows - cost per window + labor cost per/window
Paint- cost is price per/sqft, varies if painter are painting trim, primer, how many coats, interior or exterior
Sheet rock work - how many sheets 4x8 sheets will they Demo and replace
Flooring - normally charge per/sqft - find the cost of the floor you want and multiple by your sqft and add the labor cost per/sqft
kitchen - cabinets ( price per cabinet + labor to build and install) - counter top cost, price for material sqft and price per sqft of to fabricate, cut, and install
Appliances - cost of appliance + install
Bathroom remodel - depends on the size and quality finish - i start with finding the price for a small standard 5x7 bathroom and move up
Plumbing - cost depends on the size of the job and scope of work
The above information is a quick break down of how to get rough estimate of rehab cost. Make a list of the repairs needed and add them up
+++I would suggest starting with the current home you live in now. Call and ask different trade workers to give you a quote on a specific remodel to understand estimates and cost. For example call a handyman/general contractor (3 and get average price) to your house and get an estimate on a basic remodel of one of your bathrooms. Replace everything in the bathroom and bring it up to date. This will give you good baseline of a bathroom remodel cost in your area...If your roof old? Call some roofers out to get an estimate on replacing your own roof. This will give you a baseline cost of a roof repair for other projects +/- rehab cost depending on each homes sqft
Hope this was helpful.