The reason that it changes is because, if I apply it to a crop that needs all of the nitrogen, phosphorus and potassium, it has more value than if I apply it to a crop that does not need the nitrogen, phosphorus or potassium.
So really the value of the manure depends on what nutrient the farmer is looking for and I encourage farmers to speak of it in dollars per acre rather than dollars per thousand gallons because that puts it back on what nutrient is actually being sought.
If you can apply manure to a crop like corn which needs nitrogen instead of soybeans which does not need nitrogen, you'll increase its value and so the value of manure is dependant on the crop which you're applying it to.
Dr. Massey says, with regards to region to region, some areas have more productive cropland and will need more nutrients where as an area that does not have productive cropland, perhaps mostly grass, won't have as great a need for nutrients and the manure will not have as great a value.
Source: Farmscape
.