As with everyone else, what's expensive is the hardware. I always get the 8" wide shelves... I've seen the chins lay out on their shelves, and the 6" wide just seems so much smaller than 8" (but then, I'm used to seeing 8" shelves in my cages). The price of the wood will depend on the width and the length. 8" wood is going to be more expensive than 6" wood, and 10" wood (or larger) will be even more expensive. The last wood I bought was 6' boards of pine, at Home Depot... 6' x 8" x 1" was.... $5.79? Between $5 and $6.
I actually ended up buying 3 boards that were 6' long each (I needed 20 feet), because that was going to cost me $17-18 ($5.79x3) versus the 10' boards that were 8" were something like $13 each (unless I read the sign wrong)... so it would have cost $26 for two 10' boards. So depending on those shelf lengths you're looking for, it may be cheaper to get several smaller boards of wood rather than one or two larger pieces.
And I doubt you will need 30 shelves, unless you plan on making them all leapin-ledge size. If that's the case, you might. While I don't have my FN set up (yet!), I have this cage which is similar in size -
http://www.freewebs.com/nwichins/tobylarge.jpg - which only has maybe 9-10 shelves.
I buy washers in a huge pack so I dunno exactly what they run, but the wingnuts are something like a pack of 3 for $1 and the hangerbolts are a pack of 2 for $1... at least around here. The main cost of the shelf is in the hardware...