the 30% thing is silly. aside from being a gross oversimplification, it doesn't take into account the vastly different energy, transportation, and entertainment/amenity costs of different types of living and types of housing.
our electric bill per month: $50 (3 people)
our water bill per month: $0
our yard maintenance bill per month: $0
our gas bill per month: $0
our security bill per month: $0
our pool guy bill per month: $0
total cost of commuting per month: $80/person
kids want to have fun? take them to the park or the beach. walk or take transit. total cost? $0
the cost of cool stuff like computers, cameras, ipads, clothes, plane tickets, vacations, cars, etc are fixed between cities. salaries are higher in more expensive cities. after you take out housing, the remaining 50% of 100k is 50k; 50% of 80k is 40k. you can obviously buy a lot more of that fixed-price stuff - or save a lot more money - with 50k than you can with 40k.
the cost of owning a car and driving it 1,000 miles a month is about $600. the cost of a muni pass in san francisco is $66 a month - or $33 if you're low income.
take the higher salaries of big cities and the lower expenditures on many things, and the available money for rent is much, much higher than in low cost of living, low density places. and when it comes to real estate..... you get what you pay for.