Julian,

When hundreds or thousands of dollars are at stake, e.g. in Eliezer's example, or when setting a long-term policy (a key point) for yourself about whether to buy expensive store warranties for personal electronics, taking a couple of minutes to work out the math will have a fantastic cost:benefit ratio. If you're making decisions about investments or medical care the stakes will be much higher. People do in fact go to absurd lengths to avoid even simple mental arithmetic, but you can't justify the behavior based on the time costs of calculation.