How much land do you have? How much land do the comparable sales have?
The price per acre of land tends to decline the larger the size of that land, holding all other things constant.
The most expensive lots per acre will tend to be the smallest lots that will allow a single residence to be built. For example, a 1/4 acre lot being offered for $25,000 would have a price per acre of $100,000. While there may be many people willing to pay $25,000 for the initial ability to build a home, there are not many who would be willing to pay $100,000 for the luxury to build on a 1 acre lot at that same price per acre. This is because the initial utility, or benefit, of being able to build a home is the most valuable asset a parcel of land has to offer. Beyond that initial ability, each additional square foot of property has a diminishing utility, or benefit, and therefore has less value to a buyer.
The other reason why smaller lots tend to sell for a higher price per acre is the fact that there are more buyers capable of buying lower priced lots. While there may be a large number of people capable of buying a 1/4 acre lot for $25,000, there are nowhere near as many people capable of buying a 100 acre lot for $1,000,000, even though the smaller lot is $100,000 an acre and the larger lot is $10,000 an acre. So there is a higher demand for smaller more affordable lots and a lower demand for larger less affordable lots.
Surplus Land is defined as: Land that is not currently needed to support the existing improvement but cannot be separated from the property and sold off. Surplus land does not have an independent highest and best use and may or may not contribute value to the improved parcel.