How do I practice piping icing without wasting icing and money?
If you're just practicing piping, you can just pipe onto parchment paper (or a clean counter) and then scrape it off with a spatula and save the frosting. That way you can actually practice with the frosting you'll be using and not be thrown off by different textures and consistencies. 

ETA: This is what we've actually done in a few bakeries where I've worked. If you had an odd request, you'd mark a section of the counter the same size as the cake and pipe the outline or lettering onto that to check sizing/kerning etc before doing the actual cake.