I've tried these "metal" threads (hard to call them metal - they feel more like plastic). Just a warning: They are generally quite inferior in quality. The whole feel of the threads and how they handle is completely different when compared to traditional real metal / goldwork threads, they are constructed differently, the sizes are completely different, the experience of working with them is not the same (so they aren't necessarily the best choice for practice), nor will the outcome of working with them be the same.
If you want to cut down on costs for practice threads, try working solely with gilt metal (goldwork) threads, made in Europe. The gilt threads handle the same, come in the same various sizes, are constructed the same way, etc., as the higher percentage goldwork threads. They're still somewhat expensive compared, say, to floss, but with goldwork threads, a little generally goes a pretty long way.
Just my two-cents' worth...