How to import tier prices using CSV?is there any script or extension available for that?
How about Magmi Tier Price Import?
There is a script available for that called MAGMI. In order to import tier prices with it, you need to install Tier Pricing importer plugin for MAGMI, enable it, and then upload CSV with following columns and having corresponding format:
Where wholesale is the name of your group in a lower case. The value can contain both percent (like qtys 25) and percent (like qty 50, 100)
Checkout this comprehensive cheat sheet for details on mapping different fields.
If you experience troubles on setting up MAGMI and consider paid solutions, check out this one. It will do the same out of box.