Depending on the material, manufacturing may consist of injection molding, milling, vacuum-form, or some combination of 1 or more of those.
You'll likely need a CAD engineer to do a proper drawing for it, depending on the primary molding method. It's been about 4 years since I last did something like this, but the general process was something like:
1) Design and model rough prototype. This was my own time/effort and about $500 in materials (for a custom server bezel).
2) Take model to a local machinist/engineer to have CAD work done and produce initial prototypes on a Haas mill - $5000
3) Take CAD drawings and such to local low-volume production house to have initial small run of units made, at a cost of ~$250/ea.
4) (I never actually got to this stage in this project). After stage 3, you'd generally do a field test looking for whatever properties you did or did not expect from the case. Does it stay on, does it wear/damage the phone, does it survive a drop, does it affect the signal, etc. From here you'd likely have CAD work done for a mold ($5,000 - $10,000, IME) and have an actual mold made ($10,000- $100,000, IME) Depending on complexity, number of parts, tolerance, etc., this can vary widely. Once all this is done, it's usually $1-$3/unit in volume (10,000 +), but again all this varies widely on a number of factors.
Very generally speaking, you're talking about probably a $30K minimum investment to get your first 1,000 or so unit. Then you have to sell or distribute them.