My suggestion is that you ask some variation of the following:
Could you give me your own cost-to-benefit analysis of the bicycle-trail project? At first blush it would appear to have only obvious benefits. Might there also be some not-so-obvious costs of installing a bicycle trail in the city?
If the interviewee asks you to be more specific, you might consider some of the following information in framing some additional questions.
As Affable Geek points out, there are both prices and costs associated with virtually any construction project, and both are equally important criteria. We must not forget, however, the benefit side of the equation!
Wise city planners who contemplate making a change to their city's landscape engage in a cost-to-benefit analysis before moving ahead. On the one hand, the benefits city dwellers derive from having a bicycle trail cannot be gainsaid: physical exercise for an increasingly overweight citizenry, recreational opportunities for young and old alike, enhanced enjoyment of nature should the trail wend its way through a park or along a riverbank, and decreased tailpipe emissions. These are but four such benefits.
On the other hand, there are those pesky prices and costs, and while the importance of a price tag cannot be gainsaid in these days of austere city budgets, there are also costs that transcend the almighty dollar. Even a seemingly harmless bike trail through the urban landscape can have unforeseen, unintended consequences--environmental and aesthetic, for example.
Recently in my city (Pittsburgh, PA), a multi-million-dollar bridge-paving project was pushed back for several months because two peregrine falcons had built a nest high up in the bridge's structure, and they were in the process of raising some hatchlings. What if a proposed bike trail were to traverse the habitat of an endangered species of animal? Or what if the problem of water runoff is aggravated by having the trail paved with asphalt instead of gravel on top of dirt? Or what if some bicyclists insist on adequate electrical lighting on the trail, while others favor a more natural, aesthetically pleasing, and unlit--and therefore less costly--trail? After all, someone has to pay the light bill! The list of "or what ifs" could go on and on.
The point is, inherent in any new project a city may undertake are a host of factors to consider before simply plunging ahead. "Counting the cost" requires that we focus not just on what we are potentially getting from a project, but also what we are giving up. If the former outweighs the latter (i.e., the benefits outweigh the costs), then implementation would seem to be a no-brainer. However, only time will tell. What if the bicycle trail just does not catch on? Then it's back to the drawing board!