I appreciate you posing this question to the group as I am in a similar situation, with very similar numbers. I currently work for a non-profit that I've "grown up" in (first real job out of college) for the last 5 years. I've increased my salary from ~$30k to ~$50k over that time period. Recruiter responded to a job application that I kind just threw out there and now I have an interview for what seems like a great fit position that pays between $60-$70K.
I'm doing the interview, but the wife and i sat down and ran the numbers - we realized that my break even salary offer for leaving my current job is something like $62k. This is largely due to the need to buy a car and commute outside the city, whereas currently I bike 4 miles to work each way. This epiphany has made the decision much easier for me. I intend to kill the interview, negotiate a salary as close to $70k as possible and take that offer to my current employer. Even if my boss can only offer me a few thousand more/year, I win because I get to stay where I'm at (i really do like my job), keep on biking to work, and avoid purchasing a car, plus I have the extra pay (which i was intending to negotiate for anyway, based on a couple of new contracts that we just won). My only concern is in figuring out how to approach my supervisor diplomatically, so that the end result is that I end up with more salary, but without compromising our relationship.
The car thing ends up simplifying things for me, but perhaps there's a similar variable in your situation that you haven't considered. Even a slight increase in commuting distance can add up when you do it 250 days a year.