For me, it's $2.25 and 45 minutes. I would imagine that most people who live in major cities are similarly lucky. (OK, not Tokyo. There it's at least 1200 yen to get from Tokyo to Middle Of Nowhere International Airport. But I digress.)
I'm in Ann Arbor, where the only efficient way to get to DTW is by car, which means bribing a friend (still not free in terms of friend's time), taxi ($50), traditional airport shuttle (~$30-$40), or shorter taxi plus regularly-scheduled airport bus (~$25). All cited costs are one-way, and a one-way trip is at least half an hour.
Not that it's always extremely expensive, but here in the Bay Area where we have better than usual public transport by US standards, it's $8.40 from downtown Oakland to SFO on BART, or $4.75 (with a nontrivial bus transfer) to OAK. From SFO, it's also about $8 to SFO, $7 to OAK (same transfer).
Seattle now has light rail directly to the airport, and Portland has for a while. In the DC area there's National just across the river with a normal subway stop and normal public buses to BWI and Dulles in the suburbs (that stop at subway stations).
I was referring to direct subway interconnects to airports being very difficult to fund, but now I'm uncertain. I was always told growing up (80's - 90's) that the reason that reason for the ridiculous off-site airport terminals for subway systems was because of federal regulation prohibited the use of certain funds for air travel. I was always amazed to see how smoothly transport to airports worked internationally, and depressed by the difficulty in the US: Boston, New York, SF.
Since then, all of the cities have changed, and now have better direct connections from their subway systems. I presumed this was a legal change, but now I'm unsure. I can't find anything directly confirming this. Here's one of the closest I can find: http://sf.streetsblog.org/2010/06/22/bart-moves-ahead-with-o...
In this case, it was determined that BART did not comply with the Civil Rights Title VI regulations regarding equity of spending for public funds, and thus was denied the use of federal funding for an Oakland subway extension. I believe this was the basis for the earlier prohibitions, based on the (reasonable?) presumption that the poor do not benefit proportionally from having easy airport access and the rich don't need the subsidy.
I also fly into DTW a lot, same story there.