Basically, the most important thing I've seen in an air taxi is keeping your overhead low. If you do that, you will be successful, it's when you start buying hangars and vans, and trying to compete with companies like Rav'n or Grant or hell, even Refro's out west that you get into trouble. Debt servicing is a killer too. Cash flow is the name of the game - your air taxi can be profitable, but without cash moving you won't be able to pay your people on time, buy parts, or otherwise do all that stuff that makes an airline successful. I'd avoid debt - or if you do get some to buy the airplane, you need to make paying it off your number one goal, then slowly buy assets from there.
So say you finance a caravan, even with $1 mil down, you're still looking at something close the $15,000 to $20,000 / month. While the van may look like a "money maker" on paper, because it's cheaper to operate and maintain than a Navajo, if the first 10 to 20 hours of flight time for the airplane are going to the bank you're not going to be doing well without an established customer base.
That's an extreme example, but consider even a 207. Suppose you purchase a really nice low time 207 (along with a single pilot certificate) for $200,000. A steal! You're still looking at something like $1200/month in financing on a 20 year note - realistically, on a 10 year note, you're looking at close to $2,000 - that's with around $30,000 down.
If you're an A&P and an IA, the costs for you go down pretty dramatically, but that's also a pretty dangerous game in and of itself. The temptation to "git'r'dun" is pretty high, and I'd almost never recommend that road, very few people I've met are strong enough to not skimp on MX when it's their money. The common thing I have heard from every one of them is, "I'm not doing anything unsafe, it's just saving a little money." Don't be that guy.
So what's it cost to run your air taxi?
Cost per hour = Engine Reserve + Prop Reserve + Fuel + Oil + Routine Maintenance Fund + Emergency Maintenance Fund + Pilot Wages
Cost per month = Building Overhead + Staff Overhead + Business Insurance + Aircraft Insurance + Aircraft Financing + Business Stuff
You can get quotes or at least deduce for pretty much everything there except for the Emergency Maintenance Fund money and Business Stuff, and that can be adjusted for over time. For Emergency Maintenance Fund money, throwing an extra $50 - $100 into the piggy bank rather than straight into your pocket is probably a solid bet with something like a 207 - the thing I heard was no less than 10% your charter rate. The other hard one to quantify is "Business Stuff" unless you're really familiar with the community, you're going to need to market to people. Even if you know everyone, you're going to need business cards, you're going to need a website (it's 2018 ffs!) , you're going to need phones, computers, and all that errata that's actually highly important to your business.
Honestly, the Business stuff is often MORE important than the airplanes for day to day operations and (at least in my experience) constitute the bulk of a small business's headaches. You can't take reservations if your phones don't work, you can't keep track of your schedule these days without some sort of computer tracking program, I mean,you can do it on paper - but you're going to forget stuff and screw things up if you do - and I know for a fact that Rav'n and Grant and damn near everyone else is using computers so they can't possibly screw it up. Along those same lines, you're going to spend a significant amount of your time with your nose in the 8900 unless you hire someone to do that...which gets spendy.
You can lease airplanes, and this changes the equation a little bit, but ultimately it's a minor change - the whole name of the game is money in versus money out - if you can keep the money out smaller than the money in, you'll be doing ok. If you're seasonal it changes the game a bit too.
So how do you do that? Well this is where it gets tricky.
Total cost per month = (Cost per hour) * (hours per month) + (Cost per month)
You need to set your charter rate so that you make enough money to at least meet this number - ideally, unless your wife is independently wealthy, you want to exceed it. How do you do that when you have no idea how much you'll be able to fly out of the gate? This is why keeping (Cost per month) as low as possible is critical to your business survival.
You're not going to get rich doing this, but you can probably expect profits around at least 2-8% if you're smart and not incompetent and the competition isn't too cut-throat. If you can find a niche (this is the best thing you can do), you actually can make a ton of money, but you have to find an under-exploited niche.
! Also, insert old adage about making a small fortune in Aviation here 
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