"10/6" on the hat, as originally shown in the Alice illustrations and Movies, refers to the price of the hat (10 Shillings and 6 pence). This price is taken under British dollars before decimalization (British people were so bad at math, they didn't allow their currency to add up in a way that required decimals). This is the equivalent of about $105 American dollars today. In conclusion, the mad hatter was as they say "Ballin".
Another note of interest is that the mad hatter, although a reference to people who made hats and experienced mercury poisoning, becoming mentally unstable, was not that crazy. most of them would experience the opposite symptoms of that which the hatter displayed: Fatigue, low confidence, extreme shyness.
All this information would not have been gathered without the influence of marijuana.