The truth is, it does depend on a number of variables and each situation will be a little different, so in each of my examples below assumptions were made that may not always apply. For instance, the estimates do not include items that would need to be included in each scenario (pavement etc), only the differences. Nevertheless, they should form a good basis for evaluating the cost and feasibility of green options for site and infrastructure design.
Subdivision Drainage - Raingardens vs. Curb Inlets + Detention Pond(s)
This cost comparison was developed for a potential client that was developing a residential subdivision in middle TN. I was advocating for the use of raingardens and other LID techniques to manage stormwater rather than traditional drainage solutions. The developer was incredulous and wanted to see a cost comparison, so I developed the one that follows. The subdivision was a 50 lot subdivision with 1/4 acre lots. The road was 24' wide with curbs and a sidewalk on one side. The traditional drainage design included curb inlets every 250' with a cross drain and pipe in the ROW that carried stormwater to a detention pond on site. The LID design replaced the curb inlets with raingardens every 300' on each side of the road. The raingardens were approximately 300 sf in size and did not include an underdrain. Below is a table with my approximate costs and the assumptions that I made.
I did not include the cost of land that would need to be set aside for detention or stormwater impact fees, which would add to the cost of the traditional drainage system.
Parking Lots - Pervious vs Traditional
The data for this cost comparison is drawn from a small parking lot project with space constraints. Because of the size and shape of the property, above ground detention was not possible. The possible options included underground detention in the form of pipe or chambers or using pervious pavement (asphalt or concrete) and stone base for storage.
Commercial Site - Raingardens vs Catch Basins + Detention Pond(s)
This cost comparison is based on a 150 stall commercial parking lot. There is room for a traditional above ground detention pond, so that is the basis for the cost comparison. As you will see below, the costs are very close for the two options in this scenario.
These are just a few of the potential cost comparisons, but as you can see green does not always cost more. And the costs of the traditional systems above do not consider the long term costs to communities due to the environmental damage and the increased strain on infrastructure and maintenance. Those costs are difficult to measure but are equally important or even more important when considering the use of LID practices.
The key to the equation is to consider the whole cost of the systems involved, not just the individual components. Sustainability is about seeing the big picture and the long view, so its imperative that we include that in the conversation when the costs of going green are discussed.