It's $400 for a home license; $1,800 if you're a company for Mathematica. Given the work that's fine into it, if you're finding it useful and deriving revenue from it, isn't it worth it to just buy it?
More specifically, I have a log file that says I'm thinking, therefore I am. :-P
I think Descartes, if he were alive today would accept that slight adjustment. Descartes would of course then go on to rewrite Meditations II (which immediately follows "I think therefore I am") to argue that there might be an evil daemon that writes log files so that they give the illusion that I'm thinking. But if evil daemons are able to forge log files then all knowledge is impossible, so we're doomed no matter what. So best to pretend that isn't a possibility.
It always irritates me a bit that people like to throw "I think therefore I am" around as if Descartes himself didn't immediately refuted in Meditations II.