It's not so much the trees that need to be cut in an acre, but rather the trees they DON'T want cut, the terrain, and whether the acres are contiguous or not. I rarely will do by the acre. Some jobs I can stand on a hillside, look over the terrain and think "I can mow that 7 acres in xx amount of time, so here's what I will charge." The customer doesn't get a price by the acre very often. The government entities regularly send me notices for projects that are going to be bid. They often want a price by the acre with only a small window of time to preview it. Unless they are clear clutting on flat land, it's tough to not throw a number out there to bid that way. I've seen the big boys stall out on jobs that I thought would take only a few hours but because of overly thick brush or rocks or other obstructions, the job bogged down. If they had many acres at a set price and those conditions on each of those acres, it could be a real problem trying to come out ahead.
I'd suggest in the future that unless the acre(s) is fenced off and you can walk and mark the boundaries, that you give them a by the day or by the hour or week price with a realistic estimate (based on your experience on that type of terrain) of how long the job will take. Like someone else mentioned, be fair and everyone wins. I'm never the guy "makin' a killin' " and hopefully I'm not the guy "losing my butt" but rather I try to be consistent in pricing. Yes, my competitors can and do use that against me but they are in and out of business and I'm going into nearly 20 years. It just works for me but your situation might be different or maybe your properties are very similar or flat.. It's not like that where I live. Most every acre is very different and the terrain is rolling to steep across even small parcels. Good luck
