Their is a high price to pay if requirements are not defined (or not defined well). This can result in requirements defects. Requirements defects are errors caused by incorrect or incomplete requirements. If their are missing requirements or even conflicting requirements those are also considered requirement defects.
Requirements defects can lead to cost overruns, rework (which will be expensive and add to cost overruns), poor quality, late delivery, unsatisfied customers, and team members that are overworked, exhausted, and dissatisfied with their work environment.
One half of the cost of software development is spent on correcting defective requirements. Defective requirements are the most expensive kind of error to fix. Because defective requirements affect coding and testing, they tend to be spread out to multiple areas of the project which is what makes them so expensive to fix.
If you properly define requirements in the analysis phase of the SDLC, you run less risk of increased cost for the project and possible project failure.