This is an attitude that bothers me. Why should it cost anything? If it's a normal sword in all respects but disguise then we could compare it to a cane sword. Mechanically they are the same thing and the cane sword costs nothing but maybe a resouces roll or a character justification so why should the light sword cost anything? For that matter it's costing the caster a enchanted item slot already. If it costs more than the cane sword then the players are going to decide that the cane sword is easier and then you've lost a cool touch that would add to the flavor of the game for no good reason.
Because the books lead us to believe that this sort of thing is hard. They also lend toward the idea that using magic to emulate something else has few benefits. Most of the time it's something that could just as easily have been done another way, without magic, but the magic provides a sort of convenience.
Take Harry's tracking spell vs Murphy just actually investigating. Most of the time, IF Harry has something of whatever he's tracking, he can get something - though never exact and often it's a bit of a pain leading him in straight lines with complete disregard for landscaping and the like while only tracking in 2 dimensions so if it's above or below, he's mostly sunk. This MIGHT save Harry time but then he might not have anything of what he's trying to track or it may be pointing to a spot in a skyscraper with a hefty basement (possibly Undertown) and thus leave him searching 30+ floors while people are wondering, "What are you doing in my building?"
Murphy on the other hand doing investigation might ask the right someone the right questions in the first 5 minutes or find a notepad with directions scribbled onto it or something like that and be done long before Dresden, if Dresden can even cast the spell, or she might really have nothing to go off of.
Magic to emulate something else, such as skills, powers or items, don't necessarily do anything that emulated thing can't and, in fact, usually will just be another way to do the same thing. Creating an item, continuing with the sword analogy, out of pure anything, be it light, ice, ectoplasm, force, fire, or anything else would be somewhat difficult in and of itself. Holding it together through a fight would be even harder. You would almost certainly need a hilt to hold, whether as a foci for control or as an enchanted item specifically for this task. It seems to me that an enchanted item, while less universally useful, would be considerably more useful for this specific task, seeing as that's their purpose.
Bare in mind that yes, having it be MORE than just a sword is certainly something that you could do - later. Also bare in mind that Dresden is a 10 refresh character in Storm Front and by now... Well... Not to give spoilers I'm just going to say it's safe to say that by 12 books and a lot of short stories in he's probably around 20 refresh. Further, all he is really, is a wizard. He's not crossing templates with shapeshifter or taking an assload of stunts or finding faith, etc. So that's 20 refresh of I-Am-Wizard working complex magics in CREATIVE WAYS. With him even pointing out that it's not necessarily purely about power but creativity. He might have a spell or enchanted item meant to do one thing but, by being the bloody genius that he is, he can use it creatively to achieve a slightly different result.
Given that, sure a 15-20 refresh character wanting to give the sword extra abilities in the form of pure weapon skill, sure. Take refinement and pump in extra item slots. But bare in mind, Weapon:4+ is on par with grenades, RPGs, and other heavy battlefield weaponry. Every single point in this game is hefty. so going from weapon 3 to weapon 4 is substantial. That means that if you get so much as 1 shift of success, you've taken out any goons without toughness or hulking size.
Now, if they wanted to leave it as just a sword, casting a spell off the sword through evocation mid-battle in some creative manner or even pumping item slots in to allow it to do this standard or something, then I could see it as being able to do something that a sword couldn't that would certainly be cool. Let's say it's the light sword where it's pretty much focused light, right? Ignoring physics for the most part here for the sake of awesome, let's say the battle isn't going well and no super awesome sword is about to let our heroes win. They need to run. They see that. Our hero of light might be able to get away without a problem but his friends (maybe one is injured) are likely to be taken out before they get a chance to offer their concession due to initiative passes not being in their favor. Maybe the hero will, instead of conceding right away, instead cast a spell to release his sword destructively (the makeshift manner might have fallout which could damage the hilt, thus giving the hero reason to repair it with this new use in mind). Mechanically it would be an evocation spell against the previous spell holding the sword together. Say the sword's been in use for a bit, you might be able to make declarations to give you bonuses on the roll. Now if you succeed (and I would say that any fallout, predetermined as going to the hilt, would be as if you took it as backlash and thus allow the spell to succeed, just this once) then take a free action to yell a warning to your party before the held light in the blade of the sword bursts forth blinding everyone who wasn't covering their eyes. This might be an attack or more likely a simple maneuver granting the aspect "Blinded" which, when those baddies try and hurt your friends before they can run, you pass tags and make them more likely to live. Your creative use of your enchanted weapon (that is currently really nothing more than a sword mechanically) just saved the party.
Past that, if you're really needing it to be more useful, say it acts as a really good flashlight.
