First $6k/month will not get you a $1.5MM home unless you had a down payment of $500k and even then it would not cover your taxes and other expenses. A general rule of thumb for someone with good credit is a monthly payment of $3k will let you borrow $500k on a 30-year fixed mortgage. You can check this yourself here: http://www.mortgagecalculator.org/ (or use Trulia's).

The problem with Trulia's "really cool tool" is unless you use the advanced version all the inputs are wrong. For example, its tax assumptions are less than half of real cost for SF. A more sophisticated tool is available from the NY Times (http://www.nytimes.com/interactive/business/buy-rent-calcula...).

Of course getting an apples to apples comparison is pretty hard unless you're talking about buying a condo vs. renting a condo in the same building. Yes, it would be cheaper to buy a 1 bedroom in Bayview then rent a 3 bedroom in Pacific Heights. If you're looking at starter homes in a decent area, say, Noe Valley and compare those costs to renting something similar in the same area renting is generally cheaper.

The "excess for investing" DOES exist, at a minimum in the form of the down payment you did not spend. The good news for home buyers is that you get leverage on that return if the market keeps going up. The bad news is that it's illiquid and we don't know whether that's a better investment than the stock market or one of the companies we see on the front page everyday. In short, if you're never moving again buying is likely cheaper. Otherwise, it's complicated.

TLDR: It's complicated, and most calculations downplay other buyer costs and ignore other renter opportunities.