This is a good example of what was discussed below. With your equipment (tractor & moco might =$100k++), labor @ maybe $15-20/hr and fuel @ $3/gal x your gal/hr rate I would think you need to get at least $100/hr, maybe $130/hr depending on your equipment.

As stated below, if you approach many ag customers with a rate of $130/hr they will probably have a heart attack ;-)

So figure how many acres/hour you can cover with your equipment and the terrain in question, subtract some for transport to the custom field etc.  So if you think you can cover an average of 10 acres/hour  then quote a price of $12 or $13 /acre, no heart attack. 

But its the same thing as $130/hour that you need to do the job - IF you can in fact cover 10 acres/hr.  If more or less a/hr adjust per acre rate accordingly. $130/#a/hr= your per acre rate to quote.  jmho. 

Jim

Seems to me running a moco is a lower a/hr and higher hp(=fuel & wear/tear) job than some others.

Regardless of how you quote a custom job, my point is to really figure your charges on the likely revenue generated PER HOUR.