Actually, I did make a Silverwood Wand with Charged Thaumium Caps with a Frugal III Portable Hole Foci on it. But the Frugal III doesn't affect the vis cost for crafting. I swapped the very same wand into the table, with and without the focus, and the cost stayed the same regardless. I got lower vis cost by using a Silverwood Sceptre with Charged Thaumium caps instead.
This has absolutely nothing to do with this thread, but so you know, it is not currently possible to make a staff of the primal with void metal caps using base thaumcraft. The best you can do is thaumium caps. It will require the use of an add-on in order to get enough vis to craft it. The ichorium wand (thaumic tinkerer), mercurial wand (advanced thaumaturgy), or infernal/tainted scepter (forbidden magic) are required in order to craft this. It is not possible otherwise.