That sounds very expensive. Perfect invisibility is usually expensive and/or difficult in fantasy settings. Much like in the real world.
These are faulty full body invisible spells, that are heavily discounted, because they work only on the helmet. Because of the difficulty to create a full invisible spell, the majority created are faulty and basically trash, but mages sell them for low price to get back at least the material costs. Low yield.
Now you’ve got me imagining a budget version of an invisibility potion/spell/item that gives 30% translucency.
Pretty much. If you’re sneaking around you usually don’t need 100% invisibility. If it’s dark, 30%-60% is fine if you’re at all sneaky. That’s how the “chameleon” spell worked in Oblivion.