A cheap way to learn languages?

So, I'm somewhat of a hobby linguist. I eventually plan on learning a language from every major region in the world. So far, I've got English (obviously), Spanish, German and a bit of French. Looking into learning Japanese next, with Russian (or some other Slavic langauge) after that, and then we'll see from there.

But, college courses are expensive, as is something like Rosetta Stone. Is there anywhere that I won't spend many hundreds to thousands of dollars to learn a language?