Unfortunately you are the only man alive who can estimate that job... the price will vary with the depth of the ditch and the type of soil and the *risk of cutting into a buried gas or water lines, heavy tree roots, or ancient indian burial grounds, or the habitat of an endangered species. (which will require you to stop digging).
Big companies will have more to loose than you do, they will bid high for that reason.
The way we all do it, is to figure out how long it will take us to do the job, multiply the usual laborers hourly rate by 3, add in the equipment rental costs, and bid that amount if we want to be the rock bottom low bidder...most companies will bid it a lot higher.
If you think you can hire a man for $10 an hour do run the machine, and that it will take 10 days to do the job in the type of soil you have (rocks, hard pan, tree roots etc) then add that hourly rate up times 3... and the ditch witch rental cost.. and you will be low bidder for sure.
If you are bidding against a larger established company, they will be 50% to 100% higher, to cover their overhead, insurance and enough profit to make the job worth their while and cover the risks.
The huge variable here is how many feet of ditch can be dug an hour, and you will not know that until the ditch is dug.. you would have to try the machine on that ground and estimate things like tree root, rocky soil and tree root problems, which could double the time it takes to dig the ditch.
Do not forget the time it will take to backfill the ditch, you could use a shovel or a bobcat with a blade on it... if you do the backfill the customer may expect you to provide the 2 inches of sand bedding, sand over the top before back filling... if they want you to supply the wire and cable, multiply the price you pay by at least 1.3... most shops will be a lot higher, they will double their cost of materials.
Then add some money for unforseen risks, such as the ditch witch breaking and you having to pay the rental company for repairs.
Contracting is tricky business, I always added 10% or more to my final number if I thought there would be any trouble collecting full payment...
In this case it will take many days to do the job, I would probably not give a flat rate price, but would offer my services by the hour...plus all expenses... that would be entirely safe if you are sure the customer will pay you when you are finished, or will agree to pay for progress by the foot daily.
If they want a flat rate, figure the price as outlined above and then consider doubling that to cover your risks.
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If you need anything else, just let me know. You can continue asking follow-up questions at no additional cost.