when you consider that 10-20% of the bike's value should be spent on locks, factor in the inconvenience, lost time, the sentimental value, PITA, etc. this is almost always greater than replacement cost.

when a bad guy looks at the same bike, the primary concern is resale value. this is almost always lower than replacement cost.

@dontstealbikes - if you can lock up in the same place every day, consider a very heavy lock that you leave there, locked to the rack. then just carry a smaller lock, just in case. also don't forget to lock up at home.