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.