If you have not done it yet, you need to start assembling your business "team"
I would say a banker and a CPA (An attornery if doing anything complicated like a LLC) and then an outside advisor, maybe your dad, maybe an extension agent, maybe someone from Planet, or someone in a similar business in a non-compete area.
The point being, that the decisions, and what you base it on, will be made somewhat by these guys input, ie, what you are discussing above, needs too be addressed by your CPA and how you should do it BEFORE you start.
The money I have spent with the CPA has come back too us many times over in saved mistakes. I started with a "bookkeeper" and AFTER the audit, went to a CPA.
There are basically two methods to pick from, and both camps will tell you theirs is "correct", sound's to me very much like the lawncare biz.
Camp 1, go look, see what you think it will bear, factor in your rough costs in your head, take into account outside factors such as PIA factor of the client, shoot em a price and see if they go for it.
Camp 2, figure out each individual cost, do time motion studies, figure out milage, gas costs etc. and work up individual prices on each component of the job, add it together, add P&O then give a price.
I am a camp 1 guy, and think most the camp 2 guys end up just taking a more educated swag then we do, but it still boils down to a swag.
Good luck with whatever way you choose.
Remember, you can go to bed Tired, broke and hungry doing this, or go to bed broke and hungry. You don't have to do that job, and if it is not paying out, just go to bed broke and hungry. The temptation will be there at some point to cut your prices to get the work.