I'd encourage you to read about the history of hashcash, because there were various debates and implementations that might inform your ideas. I think it's a gr

I'd encourage you to read about the history of hashcash, because there were various debates and implementations that might inform your ideas. I think it's a great idea at one level, but a huge practical problem is that given people's different levels of access to hardware resources, it's difficult to tell how much a given sender actually "spent" to produce a particular hash -- which is made even more complicated by botnets (illicitly spending other people's computing power) and dedicated hash hardware (investing capital to gain a huge advantage in hashrate compared to an ordinary PC).