RSCB has guides on how to crack it. They used to post downloads for it, but decided to teach people how to do it instead. Get ready though, it's not easy.
Since RuneScape is constantly updated, neXus also has to be updated. If you want to use it forever, then you'll have to crack it over and over again, find the new download link again. To sum it up, it will be hard. Just buy it.