Keeping the cost down on any construction project today is critical, otherwise your system is likely to be 'value engineered' right out of the final build.
I can remember the days before green roof systems were common and prices were high (they are still expensive).
As someone who has much hand on type experience - in other words - has worked with many types of systems to optimize performance - fix design mistakes by others, I can assuredly share a true maxim: Green Roof system designs tend to be overly complicated. Each additional component or layer of material adds cost, sometimes significantly.
Once the green roof system is designed and specified, plants are then usually chosen to accommodate the type of trays, sacks or mats the plans call for.
Hopefully the landscape architect or green roofer in charge of the plant selection will have had some input into the type of green roofing system chosen. This is important because different species possess varying degrees of planting requirements with an important one being root architecture.
Importantly, green roof plants should be purchased from a nursery with green roof plant species experience.
Many times this is not the case though, as the green roof plants will be grouped in with the other landscape plants and selected based on a minimum amount of specification data but primarily on price. You get what you pay for.
Unfortunately, there are instances where root-bound plants, in 4" or gallon sized plastic pots will end up on the job site. The plants may have insects, soils high in degradable organic content that will decrease in volume rapidly, fertilizer pellets scattered throughout the media. They may be used to large volumes of irrigation water and sprayed with ecologically damaging pesticides (always ask about existence of pesticides on green roof plants).
Significantly, they probably cost approximately $ 3.00 - $ 4.00 U.S per plant. On a typical 50,000 SF (15,150 meters squared) vegetated roof planted with a density of one plant per SF ( or 3.3 plants per meter squared) the cost would run approximately $ 200,000.00
If you want to have a cost-effective and beautiful alternative then consider growing your own green roof plants for your project, or contract growing the required specimens with a local nursery.
Make sure either you or your contract grower are licensed in Florida by the Department of Agriculture as a licensed nursery.
Be sure to use sterile soil, soil that has been prepared and treated in an environmentally friendly manner.
Lay groundcloth down in a staging area - preferably a greenhouse, but if you don't have a green house then choose a portion of the site with similar climatic exposure conditions as the roof will have.
Use nursery potting flats - I like the 72 count and fill with your inorganic soil. The flats should be new or washed if recycled.
Purchase five pounds of Allium canadense seeds and sow according to instructions. The Ion Exchange native wildflower site is just one of many you can find doing a quick Google search.
Soon you'll have 50,000 -70,000 plants that you can let grow and acclimate on-site. Total cost, including labor should be less $ 5,000.00 - a savings of about $ 195,000.00.
The issue with this approach is it requires solid planning and coordination.
Rather than specify the plants according to the green roof system, one must specify the green roof system according to the root architecture and planting requirements of the green roof species selected.
Happy Green Roofing!