I'm a little late to the game answering your question, but I thought I'd do so anyway.
I'm assuming you're talking about residential, not commercial or industrial land.
There are several factors, apart from the obvious physical ones, like location, lot shape, topography, etc., which affect value.
The two biggest factors in the valuation of residential land are how large a property can be built, and how many units are allowed. This is largely governed by the specific zoning of the area where the land is located. An experienced real estate agent can give you a rough idea of what you might be allowed to build, but for an in-depth analysis a real estate attorney is required.
Zoning regulations vary greatly, but simple formulas such as floor area ratio (the size of the building in relation to the size of the lot) and lot area per dwelling unit, are used in all cases to determine size and number of units. Other restrictions are parking spaces, building height, yard dimensions, etc. Larger developments bring in traffic studies, drainage, etc., which can all affect the allowable size/impact of the project.
So, to boil it down, you need to determine lot size and zoning to give a rough idea of what can be built. Find out approximately what the project would cost to build, including carrying costs, legal and architectural,and roughly what the gross sell out would be. Build in a fair profit for the developer- let's say 20%, and then you can get a fair idea of the land value.
Quick example: (not figuring in parking, etc.)
10,000 sq ft lot.
Allowable Floor Area Ratio: 0.75:1
Therefore building size can be 7,500 sq ft.
Lot area per dwelling unit: 1,500 sq ft
Therefore 6 units are allowed
So, we have 6 units for a total of 7,500 sq ft, including hallways and landings, so each unit would be about 1,000 sq ft.
If each unit sold for $500,000, the gross sell out is $3,000,000
Subtracting fees and sales costs of about $200,000 leaves $2,800,000
Allow $1,300,000 for construction, and another $150,000 for architect, legal, bank fees and carrying costs. Now you're down to $1,350,000 to cover land cost and profit.
If the developer wants a pretax profit of $600,000, then the land is worth $750,000, or about $125,000 for each unit that is built.
These numbers would change with higher value units, or higher/lower build costs, but the method remains the same.
I hope this helps!