I've been thinking about this for years and puttering away at it now and again in the real world. If you have access to an asphalt-covered lot, especially one that you don't actually have rights to, how much could you do and how cheaply to make it beautiful? I think that there are some non-obvious answers, especially if you have access to a vehicle and a couple of friends. Since moving next to an abandoned area of over five square blocks I've actually been doing some slowly more ambitious jiggerings of the dynamic and maybe one day I'll write about that here but for now let's consider one thought experiment.
Let's say you've got access to a double-sized lot that is covered with parking spaces and (pretty much) ignored by the owner. We'll assume a temperate climate for now. We'll assume that the temperature never goes below 45 degrees Farenheit in the time that you're working.
So let's say that in one day you go with friends and pick up a few hundred used bricks somewhere. Perhaps you found them on freecycle. This is your starting gun since it can take months to get that cheap load of bricks. That night or soon thereafter, you all go to Home Despot and buy 80 cinder blocks and various other supplies.
You bring them all to your chosen lot.
There you mix some concrete.
You pour a broad line of concrete in a wide circle about a foot and a half wide, about an inch tall, and about 11 feet in diameter.
Along the inner line of this circle you put the cinder blocks so they will be held to the asphalt by the hardened concrete. Around four inches outside that circle of cinder blocks you put a ring of bricks. Inside the ring of cinder blocks you fasten about five or six granite "Belgian blocks".
Five of six cinder blocks are fastened to the asphalt with concrete in "random" locations in the lot. Each of these cinder blocks is bordered by one to three Belgian blocks placed a couple inches away but aligned with the cinder block to create protected nooks and crannies for plants to grow in.
In each corner of the lot you use the remaining cinder blocks and concrete to create one brick high boundary walls about a foot and a half from the actual lot line.
Then everybody goes home and leaves the concrete to cure.
The next time you can all get together you all go back and pour about an inch of rough gravel or sand into each cinder block, into the space between the cinder blocks and bricks, and for about a foot outside the bricks and inside the cinder blocks. Onto all the cinder blocks you spray "moss smoothies".
Pour soil over the whole shebang, every brick and cinder block, until they're barely visible. This will settle quite a bit.
Into this loose soil you shove hundreds of various beans obtained by the simple expedient of buying bags of beans at the supermarket. Each bean should be about two inches down and you should end up with about ten various beans per square foot.
Into this soil you also put hundreds of drought-resistant "plugs" of sedum, alyssum, and other very tough plants that propagate themselves and can stand up to plenty of abuse.
Over ALL of this, you then spread a thin layer of fine gravel. Fine enough to be the type used in fish tanks. This will allow things to "rest up a bit" and get settled and keep soil from blowing away or be forced to the side too much by things like dog pee.
Before leaving that day, water this all copiously.
Think of what this would all look like within a month. It would be elegant, welcoming, and have the beginnnings of a local ecosystem. Among other things, some kind of plants or other will "volunteer" and start growing there without your having had to put them there. All those bean vines will be encouraging the soil to stay healthy and tying it all together, literally.
The last step, just to add the final touch to bring people in, is to occasionally scatter inexpensive pretty things around the space. On the lots near me I scatter handfuls of semi-precious stones like cornelian, hematite, and clear quartz. Bought by the pound from the right stores, they're only about eight dollars a pound. Shells work well, too. Just be sure to not put them all out on one day or in one place. You will be amazed at the sense of magic it gives somebody to find something like a tiger's eye lurking under a bean vine.
Now let's itemize out our shopping list to see just how much this would all cost. I need to get to bed so I'll come back later and fill in the prices.
80 cinder blocks ($1.40 x 80) $120
200 bricks (used)
40 Belgian blocks ($2.60 x 40) $104
6 bags of cement
6 bags of sand for concrete
3 landscaper's flats of mixed plugs (3 x $50) $150
10 bags of beans ($1.20 X 10) $12
20 bags of soil ($2.30 x 20) $46
30 bags of rough gravel or sand
5 bags of fine gravel
semi-precious stones $40
transport costs $100
random tools,etc. $60
Anybody care to tell me that this will cost more than a large party?