Just my two cents: if I was trying to match the feel of soulfire in the novels, I would not allow it to be purchased on its own. It's described as "mystical rebar", and "making you more of what you are" - I just don't think that soulfire, without some other kind of magic to feed into, is capable of doing anything at all. Now, it's no Kemmlerian Necromancy, where you have to be a full-on wizard before you can take it; I'd be perfectly happy to see a character with Soulfire and Channeling: Fire, for example.
But it seems to me that your problem is that you haven't gone in with any limits on what Soulfire alone can do. Look at Seelie Magic - it's a great example of a stand-alone sponsored magic; it's got a clear thematic focus, some things it's particularly good at (like healing), and some things it just can't do (like ice or decay). You need to work out what the corresponding description of Soulfire is for your setting; it can't just be "any magic that serves the cause of holiness" - that's far too general; the normal price for "any magic the wizard feels like working" is -8 refresh. (That's evocation, plus thaumaturgy, plus two refinements to get access to all five evocation elements, in case you were curious where the number came from.)
It's described as the "fires of creation" - maybe it, on its own, works like purifying fire in evocations, and works for creation, repair / healing, and temporary constructs when used with thaumaturgy? Or maybe in your setting it's roughly equivalent to channeling / rituals for the element of spirit? Or, etc. - there are plenty of options; pick whatever is right for your game.