I think that avgas is available in unleaded - this is dyed blue. I may be wrong but this is what I heard.
The last 44 of leaded avgas that I bought came at about $330, just on 12mths ago. Which is comparable to the spiralling cost of the (supposed) 98 octane premium.
I also believe that it burns cooler than the stuff that you buy at the pump (I have done no academic study of this). The green stuff is rated at 100 to 120 as previously stated. I have found with an old race car that even 30% to fuel makes a huge difference to detonation suppression.
If I could afford it, I would run either the elf turbomax or VP sport 105 unleaded.