Just to clarify my standpoint a little, since I had someone asking me about this off forum...
1) *I* personally, as a Gamemaster, hate any and all creation systems.
Mostly because for a long time I was part of a very extended Shadowrun gaming group that acted as something of a mini-living campaign, with 4 different GMs and about 20 players who mixed and matched group compositions depending on who was around at any give time. We had some seriously min-maxing, munchy, pwoer-gaming, and outright cheating players in those groups, and a couple GMs were MUCH more lax about what they let in than others (And said abusive players, well, abused that).
Mostly this, but also partly because I generally "control" the game and power level through how much karma and money my players earn. I pace my games. And when players can bypass some of that pacing because they find a way to buy/obtain things for a fraction of what they should have to pay, I find it annoying. Especially when other characters don't have the same luxury of taking a shortcut. It creates an imbalance for me.
But this is a personal bias, purely. And is something I can deal with as a GM, because I set the ground rules. (Unless I'm stuck in a floating player game, in which case, I just have to roll with it).
2) As a writer, I like balance.
Cost. Availability. Features. These are all ways an item is balanced. So if there are ways to change up these numbers in a way that's not balanced, it bugs me. SR3's gun system was more than a little loose with it's "final cost" function, so as mentioned above it was stupid easy to create guns that were better in every way than the off the shelf guns, and yet still came out to be cheaper. It was a very bad system because of that.
So if you're going to create a system, you need to build one that replicates some version of game and design balance rather than pure "realism". If one slider goes up, another goes down. You can up the damage, but the gun costs more. Increase the ammo size, but the availability goes up. Etc. Sure, some folks can build an entire gun in their backwoods cabin out of 20 bucks in scrap metal and a $100 lathe. They're also likely to someday be in a standoff with the FBI. But that's neither here nor there. The problem is, we're playing a game, and games have rules, rules are there for a balance measure.
3) As a Designer, I too like balance, but like options as well.
And this is where things come together, IMO. The game has to be written for EVERYONE. The rules, the core rules, the baseline rules, need to be designed with every playstyle and personality and preference in mind. Which is a nigh impossible task. BUT! You can do things like include OPTIONS. I love Options. Options make my life as a Writer, a GM, a Designer, and the Missions Developer so much easier. Because it lets me pick and choose what is right for whatever I'm doing at that moment.
So for me, ideally, we create a baseline creation system that includes plenty of options, but every option has a cost. So if you want to build a custom supergun of doom, you can. But it's exactly that, custom. Which means it's going to cost an arm and a leg. This methods lets everyone benefit equally.
Then you have several options:
A) Mass Production.
YOu have a cost multiplier that is applied to the final cost of the item that brings the price calculated above down to 50% or 25% or whatever of the SUpergun Prototype cost. This represents the savings for mass production, and means the weapons is widely available to everyone (Including the NPCs. Hope it wasn't too min-maxed.)
B) Home Production
This would require creation facilities and the like, but wold let you build the gun yourself at a fraction of the price of the Custom Prototype.
There would probably be several other options as well to give further granularity to the system. But this way, the system can accommodate all player and GM styles. And none of them require the Gamemaster to work out any specifics, since they're all laid out. I think "At the Gamemasters Discretion" is often one of the laziest design "mechanics" that can be used. Mainly because EVERY DAMN THING in the book is at the Gamemasters Decision. If I want a Panther Assault Cannon to cost 16 Million Nuyen in my home game, I can do that. If they come free with a box of crackerjacks, I can do that too. It's my world, and in my world the Jets Fly Backwards.

But when you're creating rules, you need things to be definitive and solid. YOu want options to vary and adjust the gameplay, since Shadowrun isn't one of those cookie cutter game systems where everyone plays the same style.
And that's my 2 Nuyen on it. Now, I need to get to work.

Bull