Doug, For the most part, there's no real way to get an accurate idea of the costs involved here, because we cannot see exactly what is needed/planned, but I can offer a few points which might be of value.
- You mention this includes "total rehab" of a kitchen & bath; Doug, these are the 2 most expensive rooms to finish in a home, especially if total rehab includes new cabinetry & fixtures. I just completed a $70,000 kitchen remodel, and that was kitchen only, new cabinet doors but not actual cabinet replacement, with no structural work involved, and no new appliances except a dishwasher. Granted, this was using high-dollar finishes, but you see what is meant by "the sky's the limit." A typical, complete kitchen gut/remodel can often run upwards of $20,000 when including updated wiring, new cabinetry, countertops & fixtures. Typical bath remodels including fixtures will often be $3000 or more.
-The costs for the addition of central air would depend on whether your speaking of adding A/C to an existing forced air system, or a complete central air system, requiring the installation of ductwork, etc. Often this may also require updating the electrical service to the home and/or a new breaker box.
-Subfloor repairs....this all depends on exactly why repair is needed, and what's being done.
Doug, the overall cost debate of remodel vs. new construction will vary greatly depending on where you live, the rooms involved, the total size of the job, the difficulty level, etc; etc.
Get several estimates for the work needed, being sure each are including the same work & quality of finish products; cabinetry, etc. Check the market value of similar homes in your neighborhood. Decide how long you are going to remain in this home; are you planning to move in a few years, or is this where you plan to stay. What is the "median" level of finish for homes in this area; just because you might plan to move soon, you don't want to cut finish levels below what's in keeping with the area. Well-spent kitchen/bath remodel money is often readily recovered in the sale of a home.
The answers to these questions will help you decide how to approach the work to be done. Choose your contractor well; be sure you feel comfortable with them, and always but always, ask for references and follow them up. Ask to see some of their previous work; someone who's pleased with what a contractor has done for them will often gladly "show off" the work they've had done.
Doug, my first thoughts based solely on considering the situation from the info you've provided say that your $50,000 figure would not be out of line, but, there's so many variables to the situation it's really hard to make an accurate judgement.
I hope some of this helps you in your debate; post further if you have more question. Good luck