I'm still debating on if I want to source locally. It in some parts of the country it's only a $20 savings. In others it can be as much as $80. Regardless I understand the idea of more mass equals more heat retention. It does not mean more heat.
The question is does that extra mass truly make that much difference? At some point there you reach a point of diminishing returns.
The vast majority of people purchasing steel do so for faster bakes. Faster bake = more oven spring = puffier crust = better pizza (for some). Within this paradigm, the mass of the steel makes a big difference. Very roughly speaking, for a typical NY style dough (not that high oil/not that high sugar):
1/2" plate @ 550 = 3 minutes
3/8" plate @ 550 = 5 minutes
1/4" plate @ 550 = 7 minutes
Since one can get a cordierite kiln shelf that can do 7 minute bakes for around $40, that pretty much rules out the value of 1/4" plate.
As far as 3/8" or 1/2" goes, it all depends on how hot your oven gets. If you're confident that, when pre-heated to 550, your oven will run a bit hot and actually end up closer to 575, then 3/8" should be fine. Generally speaking, though, I recommend 1/2" because it's a good insurance policy. If the 1/2" is giving you too fast of a bake at 550, you can always turn the oven down to 525. On the other hand, if the 3/8" isn't quite cutting it at 550, you're up the creek.
Now, this all being said, I took a look at some of your old posts and found a reference to a four minute bake time:http://www.pizzamaking.com/forum/index.php/topic,8166.msg70316.html#msg70316
If you're already hitting 4 minutes, I'm not sure you need steel. This post goes back to 2009, so I'm not sure if you're using the same oven, but if it is, then you might want to pre-heat the oven to the max and see how hot it gets before you invest in steel. If, say, your oven can hit 600, steel is unnecessary, imo.
There are diminishing returns with steel, btw, but they seem to occur above 1/2". 3/4", for instance, doesn't seem to show any advantage over 1/2".
Re; pricing. To prevent boilover, the rim pretty much has to stay the same size, regardless of the size of the pizza. This makes smaller pizzas pretty much all rim. Non obsessives seem to be perfectly fine with rimmy pizzas, but if you're here and looking into steel, I highly recommend planning for a time where you're going to want to make authentically sized NY style pies- 16" and, if your oven can handle it, 18".
I bring up sizing in the pricing discussion, because you really want to compare properly sized plates, as opposed to a measly stock 14 x 16 plate from Baking Steel vs. a 14 x 16 plate locally. As Mary Ann pointed out, you want to sit down down at your oven and measure the living daylights out of it. Once you have that exact dimension, then start comparing prices. For Baking Steel, a custom cut 17ish x 17ish x 1/2" plate, with the additional cut down the middle (which you'll want for easier lifting) is around $140. As you can see from Mary Ann's price tag of ~$60, that's a pretty big savings.
When you start sizing the plate in a more realistic manner for authentic NY style pies, the savings increase dramatically.