If you can't afford a truly green roof, here are some other ways to reduce energy consumption.
A solar tube skylight is a reflective metal tube with a lens at the top that directs sunlight from the roof to the inside of the house. Ranging in diameter from 10 to 22 inches (25 to 55 centimeters), solar tubes can brighten up to 600 square feet (55 square meters) of living space [source: Griepentrog]. With an average cost of about $500 installed [source: DIYornot], tubular skylights are an affordable way to brighten your home and reduce your energy consumption.
Traditional skylights are more expensive to install, with estimates ranging from $600 to upwards of $1500, depending on the size and type, but Energy Star-rated models may be eligible for tax credits [source: Environmental Protection Agency].
Build a rain barrel If your funds are limited but your DIY skills are up to par, you can help to conserve water and reduce stormwater runoff by collecting the rainwater that streams off your roof in one or more rain barrels. Rain barrels can be found online or in some hardware and home and garden stores for around $100, or you can make one yourself with a salvaged plastic barrel and some basic plumbing fixtures.
Installing a rain barrel won't do anything to cool your roof, but the water you collect can be used to water your lawn and garden, and diverting the overflow from your downspouts before it hits the ground helps to keep pollution out of streams -- as worthy a green endeavor as any!
The bottom line is that there's no one "best" solution that works for every situation. While a green roof is the most expensive up front, it does provide long-term savings and quality of life benefits, especially if you live in a city where grass is a rarity. Whether you choose a green roof, a cool roof, solar panels or skylights, your lifetime costs and savings will vary depending on the size and type of your roof, your heating and cooling needs, and the location and situation of your home.
And if you still have your heart set on a rooftop garden, don't give up hope: there's some evidence that costs may go down as the technology becomes more widespread. In Germany, where green roofs are much more popular than they are here, their cost ranges from 8 to 15 dollars per square foot [source: Environmental Protection Agency.] It's still not cheap, but it's a start!