Lets say that 30 minute cab ride cost an additional $40. If your job is paying you $8 an hour then you have to work an extra five hours for it. Taking the bus didn't cost you half an hour, it saved you four and a half.
Your job may be paying you $8/hr, but if you can be doing something to move up to a higher paying job, the value of that activity may well be >$8/hr
Actually, for most 20 year olds that's not true. For me, for instance, it was absolutely a good tradeoff of time and money for me to catch a train to the airport, taking 1 hour and $10, instead of catching a $50 cab for a half hour trip (therefore, I am saying that half an hour was worth less to me than $40). Clearly he meant this 'metaphorically', but when your metaphor breaks down so easily, it might not be as meaningful as you thought. What did he actually want to say there? Was it something less pithy like 'value your time'? 'Consider the opportunity costs of everything you spend time on'?