Most people don't bid work like this as a time and materials gig. Thats where you are approaching this wrong. Most have set acre pricing and then you tweak that depending on difficulty and a few other factors.
Hills, broken up areas, multiple sites tied to one deal etc. The list goes on and on but at the time you sit down to bid any job like this or of size (multiple acres) you should already have acre pricing set.
Then it's a matter of fine tuning the bid due to difficulty, and who you are bidding it for because theres another tweak you may need to make as the work you are currently bidding often times goes as cheap as it gets.
You should be able to put a lawn care bid together for pretty much any size site in a matter of hours anymore with the satellite measuring tools and a visit an email and a phone call.