If your potential client is in a culturally sensitive field such as tourism, marketing, public relations, human resources, or politics, you have more leverage. When working with the culturally sensitive materials that these and similar fields are known for, a good translation from scratch, not just proofreading, becomes the absolute minimum requirement. A computer program will never, ever be as sensitive to cultural nuances as a real, live translator. If the potential client is a human resources department writing an employee handbook, emphasise the point that employee handbooks are filled with intricacies. These are extremely difficult to write even in one's native language, let alone to translate into another language.
Employee handbooks are some of the most vital documents a company has.
Taking chances with a primitive translation could cause enormous legal problems for the company. And your potential client will definitely start listening when you start talking about liabilities. If your potential client hasn't already bought software, and doesn't need to translate non-culturally sensitive materials in bulk, emphasize that the company will save far more money in the long run by hiring a good translator from the beginning. Whether the translator uses software or not, this will be cheaper than buying software and paying a proofreader to clean up after it. If the client buys software and hires a proofreader for cleanup, it will essentially be paying twice for something that should only be a one-time process.