EDIT: I tried out being Rapunzel, and it was every bit as wonderful as I had always hoped. See the instructable here: http://www.instructables.com/id/Surprisingly-Easy-Rapunzel-Costume/
So when my friend wanted to do a costume with me which would require my hair to be Rapunzel length, I started researching hair extensions.
Hair extensions are crazy expensive. You can spend hundreds of dollars on that nonsense. So that was out, and I pondered for a time. But then I happened to stop into a wig shop, and they had these $3 bundles of long, braided hair. For three dollars a bunch, I had to give it a try- and I'm pretty happy with how it turned out!
So here you go. How to get ridiculously long hair, for about $6.
Step 1: Materials
The darker one is more my color; I got the other one to blend and add highlights, as I'll show you in the next step.
It was also integral to the braiding process, and my friend and I worked it in in such a way that the pure blonde didn't show much.
You will also need a helper. Danger is my middle name did the actual braiding in these pictures.
Hair elastics are necessary as well.