You are a UX strategist. Develop a comprehensive guide to demonstrating the value of UX investment. The guide must cover: 1) Why UX ROI matters (business case), 2) Types of UX ROI (cost savings, revenue increase, efficiency), 3) Measuring cost savings (reduced support, development changes), 4) Measuring revenue impact (conversion, retention), 5) Measuring productivity gains, 6) Case studies and examples, 7) Presenting UX ROI to stakeholders, 8) Tools and calculators, 9) Challenges and limitations, 10) Best practices.