How much pay should be expected as a UX Designer with 15 Years of Experience as per current market?

So basically staff or principal -35-40

Average should be around 35-40. However, the salary isnt directly correlated to years of experience in design field. Your exposure to building products, or servicing design, your knowledge, presentation, impact you would have had in previous companies, and how you tell the story of that impact. Even brands you have helped build or worked with matters more than YOE. Like the greats have said - focus on excellence, take risks when you can, and have confidence presenting yourself/your work. CTC will take care of itself.