In every scenario there is a point where you value your time over the cost. On vacation is when it becomes apparent. Before when we travelled we had to evaluate whether it was more important to save money on a hotel by staying in a less desirable location or paying more to have more time to enjoy the city.
For example with tax we paid $90/night for a hotel in Vancouver. We are close to the waterfront, in downtown, and close to public transit. As we explored the city we have seen the locations of hotels we considered staying at. We could have saved $20/night if we had chosen to stay there, but we would have be unable to walk around and spent more time on the bus. With two of us, that is $10/day, and now looking back, I know we made the right decision to stay in a desirable location. Any further out, sure we might have saved $30-40/night but the 30-40 minutes on the bus would have definitely negated the savings.
So whatever you do, it's always measured against how much you value your time. Like hiring a housekeeper, gardener, etc. Or having clothes altered instead of doing it yourself. But I guess on vacation, when you have a limited amount of time to enjoy the city, the value of your time increases exponentially.
Now to decide between taking a cab or bus to the airport...unfortunately it's not just a $ choice, there are other complications involved like difficulties riding the bus with a lot of luggage and trying to transfer buses with a lot of stuff.
But how do you value your time?