i think that is a loaded question, it presupposes that there will be a massive decrease. as poplars pointed out, that likely ain't going to happen until there is a truly free & legal market. i think it is more likely that prices may drop marginally, about as much as taxes will raise prices on cannabis that you don't grow yourself.
i don't think the effect will be felt outside california as long as it remains illegal or medical only. especially as you get further away from cali, the cost and risk of transport goes way up, keeping the end product just as expensive. so yeah, probably will be bale to get more out of bud mailed east. not sure about colorado, we'll just have to wait and see.
what's to be worried about? more access to cannabis for more people is a good thing, even if it ruffles the feathers of a few members of the community who may profit more off the currently quasi legal or illegal status.
about a 95% chance that it will based off the most current polling. PPP (public policy polling) just ran its latest poll with over 2000 likely voters (sample sizes up to this point have been 500-600 or so)....yes on prop 19 was ahead by 52%-41%, with 7% undecided. not only is yes on prop 19 ahead, but it is showing a divergence that favors yes more and more.
http://en.wikipedia.org/wiki/California_Proposition_19_(2010)#Polls