I would make the distinction between what is code required for new construction vs. what it would take to bring an old building 'up to code.' New construction is pretty straight-forward. Here is the standard, meet it. In the circumstance of the property owner who wanted the house he just bought 'brought up to code' that is sort of his choice. At least here in Louisville, if your installation met the code at the time it was constructed, then you are 'compliant.' If you choose to upgrade, then you can pick and choose what and how much you want to upgrade. The simplest example would be upgrading to GFCI in the bathroom or kitchen. It can be as simple as just replacing the receptacle (which doesn't even require a permit here) and your system is much more compliant than it was before.
Ironically, many of the expensive upgrades we see come through, at least for single-family residential, are mandated by the buyers insurance company, not us. A system may be compliant as far as we are concerned, but if a home inspection reveals potentially hazardous conditions, the insurance company may require an upgrade. An example of this, in my own experience, was an insurance rider that required an old split buss panel be replaced (circa 1977 vintage) with a new service panel. Why? No main disconnect, plus the breakers themselves were old and in poor condition. As far as Metro was concerned, if there is no hazard, the installation is fine, no matter how old. The insurance company had, well, other ideas.
In new construction, the upfront costs for AFCI, tamper resistant outlets, etc, seem to be rather modest as a percentage of the total cost of the home, but that's debatable I guess. Depends a lot on the price of the home.
Personally, if I bought a house that I wanted brought "up to code" I would want it solidly grounded, a good modern service panel, inspected for potential arc faults/leaks to ground and maybe an AFCI for the bedrooms anyway. I would want GFCI for the bathrooms, kitchen, etc, and a GFCI outlet on the front and rear of the house outside. I really doubt that this would cost all that much.
