Earlier quoted context omitted.
The best approach is not to do it. Demanding money from someone that didn't hire you is never ethical - just childish. Would you like it if I showed up at your house, mowed your lawn, and then started banging on your door demanding $100 for mowing your lawn? Also, what marketing value - if you're just pwning random web sites rather than getting hired to test a site's security you aren't in any market.
The grass in the lawn may not be that dangerous to other people. However, if your house is emitting radiation, and a hero breaks in to clean it up for the sake of other people you service, (because the town does not need to wait for you to hire someone) the hero deserves a reward and the owner of the house deserves punishment.
That's modern society for ya.