Sponsored Magic gives Channeling and Ritual, which alone would be enough to justify it's usual cost, then tacks on a few minor benefits.
Sponsored Magic is somewhat suboptimal when you already have Evocation and Thaumaturgy, by design. Could still be useful, though, as an alternate option at high refresh levels.
To be honest, Seid Stones is probably fine as it is. It's just that it feels rather... well, weak. But it's fine as it is. I might make it so that it gives a free focus item slot as well, but that's more to do with my preferences than anything.
The Resource Collection isn't canon... and I did say [-3] was a common intepretation. But anyway, if you want my opinion, I cost the Swords of the Cross like this: [-4] for ACAEBG, [-1] for Divine Purpose, and everything else (yes, even holy) is free. After all, being holy is more of a side effect than anything inherent to the swords.
Also, Soulfire doesn't quite do the same thing. It's Channeling + Thaumaturgy for a total of 5 refresh, same as Sponsored Magic. It's extra benefit doesn't fulfill Catches - it reduces all toughness levels by one - so it's mathematically less than the ability to fulfill the Catch, with the exception of Inhumanly Tough creatures. It is slightly more cost-effective than ACAEBG though, in both sense.
And if you're open to having the Catch be anything, I suggest whatever makes it more hilarious for your group.
You can't go wrong with the Rule of Funny.