It's not an answer we can really nail down, David. It's kind of like wondering what round-count we can expect in gunfights: once the shooting starts, it will range from one shot fired to more than a hundred, and we won't know the final tally until it is over.
I've seen self-defense shootings that literally didn't cost the shooter a penny: the case was quickly ruled justifiable by the Criminal Justice system, and no lawsuit was filed in the Civil Justice system. On the opposite end of the spectrum, I've seen self-defense cases that cost six figures to defend in criminal court, and have seen seven-figure settlements and judgments in civil court. Lawsuits tend to drag out much longer than criminal cases, and that obviously runs up the legal bills.
As a rule of thumb, good defense lawyers on the criminal and civil sides alike will be charging hundreds of dollars an hour, and there will be a great many hours involved. There are also fees for private investigators, document copying (often at $3 per page for hundreds and hundreds of pages, not the 15 cents per page you'd pay at Kinko's), expert witnesses, depositions of the other side's witnesses, flying in material witnesses and putting them up in hotels, and more. A Public Defender won't cost the defendant, but few of the productive members of society who frequent this forum would qualify for Public Defender assignment.
The quote you mentioned of 20 grand to cover the defense sounds cheap, until we get to the operative term you quoted, costs going up "as the shooting becomes more problematic." As soon as the shooter is indicted or sued, the case just DID become more problematic.
Self-defense can be expensive. But allowing ourselves to be murdered is so much more expensive in so many ways, defending ourselves on the street is still an obligation.
The discussion points up something you yourself have made clear in many of your posts over the years on GlockTalk, David: if avoidance is at all a reasonable option, it is the best strategy...and the theory of "I'll worry about preparing for the aftermath after I've had to shoot somebody" is NOT the winning strategy.
Thanks for bringing up an important question,