I would imagine the time of year will effect this as well. If your trying to get it done in the fall or spring when farmers are having all there fields limed I would imagine the price is higher because they are busy and probably cheaper in the summer when not a lot goes on in ground prep.

Also your area probably has a lot to do with it as well depending on soil types. There is a LOT of clay in my area so it is very common to lime fields here.

My pond renovation thread here