In my effort to sort out my closet, I took a look at what I wear the most, which is work clothes. I work in a business causal and sometimes business attire environment so I have some flexibility. I find these pieces to be stables and can fit both formal and causal work environments, so I think they are worth spending a little extra.
Trousers - a couple of pairs of trousers in black and gray are essential. I would leave brown or tan to slacks. These will have to work with the blazer, which I will talk about. It would be nice to get machine washable wool, so you don't spend a ton on dry cleaning. A good fitting pair of trousers will can also be a part of of night out outfit.
Pencil Skirt - I would also get one or two pencil skirts in gray or black. One of them should be the same material as your trouser and blazer.
Blazer - One good blazer will take your trousers or pencil skirt from business casual to business attire. Make sure that it's the same material as one of your skirts and trousers - this way you have 2 suits without buying 2 suits. A single breasted, one or two button blazer, with minimal hardware details will stand the test of time. Make sure to have the sleeves hemmed.
Coat - Depending on you local climate, you will want a coat with the right weight. A good coat will last you a long time so be sure to get one in a classic cut and color.
Bag - Same as the coat. Better to spend a little more and get more year out of the bag then spend less and having to get a new one every year.
They don't have to be a designer label or be very expensive. Usually, places like the Limited, BR, J. Crew, Ann Taylor have sales that can get you at least first 3 of these for less than $500 total. Of course, this list to taking into account that you don't our grow the size of these clothes. More reason to stay in shape. =)