My s8 has crack screen and back can i replace my device for new or used s8 how much would it cost?

If the screen break has not damaged or touched power slot then it will cost you $189+taxes otherwise $230 +taxes

Go to a ubreakifix they are certified by Samsung and doesn't cost much and very fast

If you can, go directly to the Samsung showroom at the Washington Street address. Technicians are on the second floor and can evaluate your situation