Mathcad 15 is free! But you must buy Prime. (Mathcad 15 is required to translate older, version 15 and earlier, files into Prime.) The same license activates both versions.
I love MathCAD. It's great for scratch calculations, unit conversions, and doing the math of calculations you have to do by hand. I've used it in a Computer Aided Engineering class do the background Mechanics of Materials Calculations in order to prove the SW Sim. Also useful for stings of problems where one/two variables change.
Note that the 'price for Prime' is a subscription, that is: a yearly fee!