I wrote this a few years back for someone wanting to do the same as you, maybe it will help you too.
Real world costs are a hit and miss thing based on what you bought to start with. Keep in mind that I haven't flown a Cessna in about seven years and never owned one but we'll ball park some numbers for you so you have a starting point. If an actual Cessna operator post up with their numbers go with them. I'll "worst case scenario" it for you though.
Couple of points to start with, money wise;
Purchase of airplane - $50-80,000 so 20% down is @ $15,000 w/ $900/mo note for 7 years, +or-
PLUS sales tax for your state. Maybe 7% @ $4,000
Insurance - probably gonna be around $8-10,000/yr until you get some time insuring for $80k on hull
AND you'll have to pay around $500 per operator you work with to cross-insure and that's IF you both have the same underwriter. It can be more if they are different.
Liquid cash - going into this thing you should have another $10,000 liquid cash in start up capital for training fuel, hotels, meals and minor maintenance issues. Remember that everything on an airplane costs $1,000 period. Also if you have an accident you'll need to pay your deductible so it's nice to have that ready.
Operating line of credit - To insure your success you should have an operating loan established for around $50,000 in the event of a major mishap like the engine making metal or hitting something with a wing, prop strike, etc. All that shit will cost between $15,000 to $30,000 and all the while the plane is down not making money you'll be making payments and feeding yourself.
Bottom line is you should have around $40,000 CASH right now and an operating line for another $40,000 - $50K set up BEFORE you go buy anything.
You will most likely use every bit of the cash getting going but if everything goes right you won't need much of the operating line. BUT it's nice to have to ensure your success.
NOW hourly costs - with 17 to 18 gallons an hour fuel burn.
Around $125.00 to $150.00 an hour with you paying for fuel and NO pay for you. This is dividing your fixed yearly costs into about 200 hours, paying for your hourly overhaul expenses for engine, prop and paying for all of the "little" miscellaneous airframe and/or spray system parts. GPS fixes, software upgrades, spare nozzles and check valves, pump seals, landing lights, oil changes, brake pads, tires, gate seals, rod ends, load hose fittings, air filters, windshield cleaner, rags, push to talk switches, radio work, 100 hour inspections, etc.
Production rates -
Things to take into consideration would be your need to EASE into this thing.
I'm gonna say CONSERVATIVELY starting out (first 50 hours) in unfamiliar country you shouldn't be flying more than three hours a day. Then no more than six hours a day for your first season after you get familiar with the area and comfotable with the plane.
It is reasonable to think that you could haul 120 -150 gallon loads to start with so at 2 gallon work you'll be hauling 60-75 acre loads (of course if your operator is doing three gallon work this will cut your hourly gross greatly). Depending on the field size and ferry distance you could probably do about 1.3 loads an hour so around 90 acres an hour.
Don't really know what people are charging where you are thinking about going but let's ball park.
You would be around 55%-60% of gross with the plane, they pay for fuel. Say $8.00 an acre, you'd get around $4.40 an acre. $4.40 x 90 = @ $400/hr gross for you about $750-775 total gross. Less per hour production would be less for you, less per acre cost would be less for you. The important thing starting out is that you will need to discipline yourself to not get focused on hourly production for a season and just stay alive and learn how to do clean work. That is hard when you have payments to make!
NOW let's look at from the "whole picture" perspective.
Fixed costs per year - insurance, annual, payment @ $26,000 / $400/hr = @ 60 hours NOT including what it costs to fly those 60 hours.
Other first year start up costs - @ $20,000 = another 50 hours
Food money for you - @ $30,000 = another 65 hours
Costs to fly those 175 hours - ballpark another 50 hours
Total about 220 hours or about 20,000 acres @ 90 acres an hour.
That is to pay for everything AND pay yourself something.
Remember that after a bit of taxes this is Ramen Noodle country on the remaining cash after you make a pickup payment and other various life expenses.
Other thoughts on this - right now the credit market is fairly tight and you'll be putting down at least 20% on the value of the plane and they aren't wanting to give out operating loans on new businesses so you'll either have to have some "stuff" to secure the loan with or a real rich daddy to co-sign for you and put their shit up for you.
You'll need to have fairly good (680 or better score) credit to secure the loan on the airplane. But they will usually use the aircraft itself to secure it.
The main problem being that it is a work airplane and the bank views that differently than a play airplane. It seems backwards to me but I'll give you an example.
My "play" plane bought with 10% down on a 20, yes 20, year note at around 6.9% fixed rate. It's like they're giving them away!
My "work" planes are 20% down on a ten year (and I had to really push them to do that, normally not more than 7 years) variable rate note WITH a life insurance policy to back it up (life insurance on ag pilots is about $170-$200 a month for $300,000-$500,000 policy).
You will also be looked at as a start up business and with no "sure income" it'll be a stretch for them to write you so focus on SBA loans. A real pain in the ass and more expensive but may be your only option in this case.
Last thought is that you will probably have about $4,000 in business start up costs. Accountant, Lawyer, etc. You'll need to start an LLC. probably and get set up with a workmens compensation policy even if you do like most of us and waive your right to coverage (make sure you have medical insurance that covers you IN THE AIRPLANE if you do this, I don't want to pay for your medical bills if you have an acident) you still need a policy and that'll cost a few hundred bucks. Most operators will require you to provide a copy to them prior to subbing to them. Make sure you do this for a liability stand point. They may still sue you if you go through a school bus full of kids but it'll buffer your personal assests if you use a corporation to work through.
There is more to this than I think you are thinking. Not insurmountable by any means but "buying an airplane and going to work" has more to it. You can do it with a little less but I think you will succeed if you prepare like this.
You haven't even asked about ground equipment or hiring an employee for later, that's a whole other pain!
Another side note is that ten to fifteen years ago Cessna's were flown on boll wevill contracts for $320-$350 an hour flat rate GROSS. That included fixed costs, hourly operating costs, fuel and pilot pay. Many guys made money at that but it was a lot of hours to divide your fixed costs into, like 350-500 for a season. I don't think with what fuel is and what airplane parts cost you could do that any more. Probably $400 minimum. More if you were only going to fly 200-250 hours.