Final answer:
In the context of developing an Information System, building a custom system usually incurs the highest cost and takes the longest time but can lead to a competitive advantage.
Step-by-step explanation:
When developing an Information System (IS), the method that generally takes the longest time and has the highest cost, while also potentially providing a competitive advantage, is building your own custom system. Building an IS from scratch is typically the most resource-intensive process because it involves in-depth requirements analysis, custom software development, system testing, and ongoing maintenance and updates. This approach allows for a tailored system that can meet specific business needs and help distinguish a company within the market, potentially leading to a competitive advantage.