The Wolfram Mathematica for Students costs around 125 $. This is rounded because on the official website, you'll see it listed in GBP (Sterling pound). However, if you access it from USA, prices will change accordingly, but the total amount will stay at that level.
To buy it, access the following website and pick the version you choose fit for your needs. You will see some price variations and each package contains additional goodies like support for years, email support, 2nd partner license sharing and more.