I started an Aerial Photography company in my home city. Takes about a year to get word out then just repeat business from Real Estate agents. We usually schedule one flight a week to make sure we have multiple targets to shoot per trip which keeps costs down. In an hour of flying you can shoot 3 or 4 houses if they are spaced near your base. My minimum charge is a little more than $200. More $$ for more extra angles, more prints and special requests. I use FBO aircraft and flight instructors when I'm out of town (even sometimes when I'm in town).
It does take some cash to get started (quality digital SLR, large format screen with speedy computer [apple], High quality large format photo printer [epson] and a FBO where you can get a good rate on an aircraft)
It's worked great for me, been at it for 4 years now.
Just stay out of my city. I don't like competition.