I have used California Closets and had them create a built in work station in an area I once had a typical closet (with two sliding doors) in. The closet doors were removed, and I now have a white (melamine?) work station that is about 6 1/2 feet wide. On the left side is a bottom filing drawer, an open area for my TV, electronics, and above the TV opening are shelves with doors that house my printer. I then have a desk with top and grommets, keyboard tray, and to the left three drawers. Above the desk I have a combination of open shelving and shelves with doors. I paid a bit over $3K for it, and that was at least five years ago. I am happy with my system, and thought California Closets was a good company to work with.
I was never given a per sq ft price when I did my research before installing my system. I had some unusual issues to deal with, such as a sloped ceiling on one side of the system, and a brick chimney on the other. I imagine companies factor all those kinds of things in when determining the project cost.
My parents have California closet systems in every closet, their pantry and even their garage. I think they dropped over $20K for it all.
The cost for closet systems really varies widely depending on what you want and what materials are used. I would get bids from several companies, and then decide who you are most comfortable with.
We are undergoing a major home renovation right now and my general contractor used to have a custom closet company, where he installed more high end closets using more costly wood components. I know when I am ready to have the rest of my closets done, I will be giving him a call to do my closets as he still has lots of material and has the expertise to do it.