Probably the best is to do your own survey in your marketing area.
Call and ask to interview some of the prominent portrait studios in your area.
When you visit their studio, you will see what kind of equipment you will need and in some cases what you will NOT need.
Be sure to ask what percentage of their work in the studio is "commercial" and if they rent out the studio to help defray the costs of operation. You will also want to ask what percentage of their work involves weddings and other events.
There are the fixed and variable costs you will need to include in your P&L sheet.
Capitalization:
Camera system: This can cost anywhere from $50,000 to $150,000 depending upon if you need a medium format digital camera.
Building a studio in an existing light industry suite ($25,000 or so, depending upon how elaborate your electrical, kitchen and dressing rooms are.
Studio flash: Depending upon how many different "sets" you will be using at one time, you will need from two to three power-packs and assorted heads and light modifiers.
Office computer, production and off-site notebook ($5,000 + the licenses for at least two Photoshop CS4's)
Of course there are many more pieces to this and taking tours of some of the other local studios will fill in a lot of blanks.
NOTE: Do not forget to include in your operating budget, setting aside enough money to replace your professional camera bodies every 18-24 months and your computer about every 2 to 3 years and software upgrades as well.
