Actually, inflation would raise prices. Given the figures cited in the article, $10 in 1980 should give something like $20-25 now based on the US CPI. Thats precisely why it's interesting - the real (vs nominal) price has actually fallen since 1980.
And then you're right, basic supply and demand theory would suggest that supply growth has outpaced demand growth over the same period (and by a rate about the same as the inflation rate to keep nominal prices constant). But why? loosening regulation/enforcement is mostly on the demand side, I would say. It's probably getting easier to grow higher quality pot (more information readily accessible, better equipment, mobility, economies of scale, etc). The overall market likely behaves perfectly competitively (its a commodity, no single dealer could charge more per unit of 'high' without losing business). So, perfect competition with supply growth = lower real prices.
And then you're right, basic supply and demand theory would suggest that supply growth has outpaced demand growth over the same period (and by a rate about the same as the inflation rate to keep nominal prices constant). But why? loosening regulation/enforcement is mostly on the demand side, I would say. It's probably getting easier to grow higher quality pot (more information readily accessible, better equipment, mobility, economies of scale, etc). The overall market likely behaves perfectly competitively (its a commodity, no single dealer could charge more per unit of 'high' without losing business). So, perfect competition with supply growth = lower real prices.