Good question. The answer to that should be related to the answer to why things change in price.
Reasons for a smaller dollar amount: inflation, cheaper production, too few buyers, too many sellers (thus more competition)
Reasons for a bigger dollar amount: more expensive production, too many buyers, too few sellers (thus less competition)
I'm no economist but I suppose the reason should be a combination of the above. However, my intuition would expect weed gets cheaper over the years due to inflation and competition. So there is probably another reason: It might have to do with weed culture itself, which is highly ritualistic I think. The ritual (of consumption) probably doesn't change and the price doesn't change either.
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.
Reasons for a smaller dollar amount: inflation, cheaper production, too few buyers, too many sellers (thus more competition)
Reasons for a bigger dollar amount: more expensive production, too many buyers, too few sellers (thus less competition)
I'm no economist but I suppose the reason should be a combination of the above. However, my intuition would expect weed gets cheaper over the years due to inflation and competition. So there is probably another reason: It might have to do with weed culture itself, which is highly ritualistic I think. The ritual (of consumption) probably doesn't change and the price doesn't change either.