PoE allows end game currency to drop anywhere so there needs to be a penalty for farming content you are way overlevelled for. drop penalty kicks in when your level exceeds zone monster level by more than 2 levels. the penalty is 2.5% per level after that 2 level buffer. so if you are level 30 in a level 20 area you'll see a 20% decrease in currency drop rates. There is a roof to this however. Level 68 is the maximum considered character level. At level 100 you are still considered level 68. So that means no matter your level you will always have a 100% drop rate in zones level 66 and higher. ie the map system.