If you purchased Mathcad 15 after January 2011, then you can get a license for Mathcad Prime 1.0 for no additional cost.
Your current license should work for Prime 1.0. You can download the Mathcad Prime 1.0 trial version at http://www.ptc.com/products/mathcad/mathcad-prime-1-0/free-trial.htm
If you have an older version of Mathcad 15 and are on a maintenance agreement, then you can upgrade to Mathcad Prime 1.0 for free. Both versions can run simultaneously on your computer.
If you go to www.ptc.com/products/mathcad/ and click on How to Buy, you'll see the various upgrade prices for people who do not have maintenance.