Ran a campaign once where the 80% attrition rate of magic was, literally, a problem. Magic was leaving the world every time someone disenchanted something and scholars had calculated there was a critical point past which all magic would simply cease functioning. Since all transportation was based on magic, this was bad (most modern major cities function solely because of mass transport, two days without food shipments and the city is starving to death. This is the equivalent of all gasoline suddenly disappearing).
So during levels 1-10 the PCs researched/found/re-built a Magic Aura Gyro Indicator Control, or M.A.G.I.C, that, given sufficient magical energy, could detect the exact level of magic left in the world. They were actually doing this to find magical artifacts and sell them, little did they know...
Bad news, world was teetering near the critical value. So the first five levels of Paragon were going around to kingdoms and explaining that they simply could not afford to disenchant anything. Magic Item prices and Residium prices spiked, people stopped casting rituals, economy had issues. I spent pages and pages working this out economically. A couple of wars happened on kingdoms that wouldn't stop, some old feuds getting worked out there.
Second half of Paragon was researching where Magic came from in the first place, so they could get more and solve this issue. Interesting they found multiple sources. Death of Gods, cataclysmic events where planes temporarily merged, deaths of various planes, the materialization of pure energy from various monsters, an extinct species whose excrement was residium (all eaten by Rust Monsters, tragic...).
They discovered the Law of the Conservation of Magic eventually. All that Magic didn't just disappear, it went somewhere. 21-25 answered the where and how. 26-30 was fixing the issue... temporarily, at least.
And, of course, they could detect nearly anything they wanted... but even a simple transfer ritual costs some magic from the world. If you work through all the implications, which took me forever, you get a really interesting dynamic without changing anything else about 4e's economy.
This was for a theme party: All Arcane characters, so it really engaged the party.