Nothing is set in stone with respect to characters until the saga starts, and even then, I'm more than amenable to change things that make no sense. And my ruling on Personal Vis Source stands. If you use it during advancement, it's 3 pawns, and it's expired at saga start. If you want it at saga start, you don't use it during advancement.
So, the fee to do the ritual is 24 pawns, and you still have to supply 12 pawns of vis for the ritual itself, for a total of 36 pawns of vis per casting, which is inline with the costs of a verditius making an item. Granted, it's not seasons of effort like a Verditius, but it is extremely risky. Consider that in this saga, rituals are always risky, and this ritual has 13 botch dice. It's nearly trivial to drop 3 botch dice for a familiar. Likewise another 1 for mastery, which makes it 9 botch dice. Getting below 6 botch dice is going to be the challenge. True, more mastery and Cautious Sorcerer and a better familiar Golden Cord can further reduce these, but let me point out that such a person is rare, exceedingly specialized, and is capable of charging whatever price he wants, basically recouping all of his investment in strengthening his familiar bond, studying the mastery ability to the exclusion of other things and perhaps acquiring or already having Cautious Sorcerer.
As you say, there's really no reason someone couldn't have invented the spell. Indeed individuals are more than welcome to invent the spell themselves and cast it upon themselves, taking on all the risk personally. Doing so, they even knock 5 levels off the level of the spell, reducing risk of botching just a bit...