Why is AVGAS so much more expensive than car gas?
The LL of 100LL stands for Low Lead as it has tetra-ethyl lead added as a detonation inhibitor for high-performance engines. Despite the "low" moniker there actually is a lot of it in there, and it's toxic. This fuel is expensive for a number of reasons:


The lead itself is expensive
The lead additive means that a refinery must shut down and clean the lead before producing other fuels, this means more refinery time and therefore more costs
Lead in fuel cannot be allowed to contaminate other fuels so you cannot pipeline leaded fuel. It has to be transported in containers, adding to shipping costs
There are extra controls around the lead in many parts of the world, namely storage requirements and paperwork, all which costs
100LL fuel has many more "aromatic" hydrocarbons than mogas (auto fuel) in order to increase the octane levels and prevent fuel from vaporizing in your lines at high altitude. It's much higher grade, so it costs more
It's a specialist fuel made in much less quantities than other fuels, so a premium is added to ensure a profit


Keep in mind that the Avgas is taxed less than Mogas in most places. It would cost even more if it were taxed the same! 

Efforts are underway to come up with a 100LL replacement that will work the same in all engines and situations, which is a hard problem to solve. Whatever fuel they come up with, while having no lead, will still likely be more expensive than mogas just because of the more expensive hydrocarbon mix and the specialist nature of the fuel.