Not a really simple answer there. Sometimes you may have only 100 acres worth of boundary in a 200 acre field, like one big pivot. Other times, you may have drainage ditches, off cropped zones, etc. inside the field. I used to charge based on the acre period, but refined things a little on that. I now charge by the acre, plus add $5 per "object". By object, I mean a pivot point, wet bottom, or single "quadrant" separated out in a pivot field, parking pad for a pivot, other things like that. You really need to map those out, and all of that takes extra time. As for the charge per acre, well, depending on who and where, $0.50 would be a good deal for a good job, and $1 would be a little on the high side. Most will charge on gross acres of ALL objects mapped.

Another thing would be field versus planted acres. My standard procedure is to map PLANTED acres, as that is how I charge for scouting, for example. Some landlords require you to pay rent on the ground you turn around on, too, as you are using it. As for buying fertilizer, for example, I know some farmers that don't want a grain landing in the fence row, so they order based on my mapped acres. Others, to make sure enough gets in the field they plant, want to overlap a little into the fence row and tree line, so they order extra based on my acres. I think it would be worth a premium to map field and then to map planted acres; of course FSA has gross acres on their maps. Depending on how wet or dry the season is, this could certainly vary from season to season. I have been paid to go in and measure "net standing acres" before to help a farmer get rent adjusted for those acres that drowned out.When I do this, I download backdrop aerial images, that is current MrSID's, and provide printed copies as well as computer PDF's on all fields, with borders, names, and acres printed on them.

It takes a lot of time to really put together a good set of field maps, but is something worth doing right. This really gets the landowner and farmer on same page of field acres. I provide an additional farm by farm, field by field printed report listing field acres, planted acres, off cropped acres, net irrigated acres, etc.

If you have yield monitor or spray log records available, many GIS programs could calculate net acres off that data.