here is a ball park for the material from the place I'm looking at.
30x48x9
4x6 pole, 8 o.c.
1 bag of secrete per pole
2x6 skirtboards, 2 rows
2x4 side nailers 24 o.c.
2x4 roof purlins, 24 o.c
Trusse 8 oc 3 5/12
I think 29 ga metal.
Insulation, 1/4" Fan Fold on roof.
2 ag winds, 3x2 with screens
1 36 entry door.
2 9x7 overhead doors
Approximate cost $7K.
Wedge
30x48 is a good size.
4x6 post will work, but 6x6 are better. They stay straight better, and will be stiffer. Especiallly 8 ft apart.
How deep do you have to go for your posts? I'd budget three sacks per hole, but hope to use just two. Holes will need to be three feet deep at a minimum with four feet being better in some climates. find out what is needed in your area.
Why do you want to use two 2x6's for your skirt, or bottom boards? The cost of one 2x12 is the same as two 2x6's. I'd base the size of this board on how much change in height you have around the barn.
You call them side nailers, I call them purlins. Going 8 ft with a 2x4 is going to give you some movement in time. I would use 2x6's that are 16 feet long and overlap the ends when attaching them to the posts. Put them up every four feet if you are using metal for siding. If you are using wood or a sheet product, then go 24 inchs.
Same thing with your roof purlins. You'll never get 2x4's on their edge to last with an 8ft span. I'd use 2x6's that are 16 ft long and overlap the ends. Put them every four feet if you are using metal R panels.
I wouldnt' use the thin metal. 26 gauge is prett good and if you only want to do this once, it's worth the extra money for the thicker metal. The metal is going to be more then you expect. Not only do you buy the panels, but you need screws for fastening the metal to the wood, and the metal to the metal. If your pitch is shallow, you'll need to seal the overlap. If it's steeper, 4:12 or more, it's not needed. You'll need edge trim, ridge caps and vents. Gutters can be added later, but are a good idea if you can keep them cleaned out.
Shop around for windows. We have stores that sell regects and other stores that sell used windows. Allot of the time you can find brand new windows for half price at the lumber yards where the contractors buy from. Either the homeowner changed their mind in what they wanted, or somebody messed up on the order. Either way, if your not picking on size, color or style, you can get some very good windows for a big savings this way.
36 in walk through door is a good idea.
9x7 shop door is just begging for regrets. 9ft wide is tight, but doable if that's all that will fit. 7 ft tall is going to be a huge mistake. That's how tall a standard residential garage is, and you don't want that in your shop. With nine foot ceilings, you can go 8ft for the shop doors. That's still too small, but if you are set on the low ceiling, that might limit your door height.
Where will the shop doors go? If it's at the gable ends, then you can go up another foot and have the opening at the very height of the celing if you put in a roll up door. Given a choice, I'll always pick a roll up door over any other style.
For you ceiling height, why 9ft? Are you thinking that your posts will be 12 ft tall and you'll put them in 3ft? While this would be the cheapest way to go, it's also going to be just a little on the low side, and a source of frustration over the years. You can build this as cheap as possible, or you can build it as something that you'll get good use out of the rest of your life without any regrets. I'd buy those 6x6 posts 16 feet long. This will give you 12 feet to the ceiling. Trust me, that's not very tall, and for a really usable space to work in, 12 ft is ideal.
12ft to the ceiling will allow you to have ten foot tall doors!!! This will let you drive just about anythng into your shop that you will need to work on. My door is 10x10 and it's just right. I had one in my previous shop that was 12 ft wide, and that was easier to back my trailer into, but not such a huge difference that I went that big again. With ten feet wide, I have just a big more wall space for my welder and garbage can.
You'll find that no matter how big you make it, you'll soon use up every inch of it and wish you had more space. If it's getting too expensive, you can always do the posts and get the roof on this year, then do the walls the following year.
I always tell my clients to tell me what they really want. Not what they think they can afford. Then we work on what it will take to make that happen.
Eddie