The price of land  fluctuates with the supply and demand market. The selling price is obviously set by the owner of the land. The owner may have estimated it themselves or with the advice of real estate experts, such as a real estate agent, or after consulting a land surveyor. The owner of the land can also call a property developer who defines the current value of the plot and its future value in terms of the possible use of the land and the income that can be derived from it.
Note that the sale price is the amount proposed by the seller, but the amount that the seller and buyer agree on when the sale is finalized is the transaction price.