I was wondering, if the reward goes down to 25 BTC instead of 50 BTC per block, then that will drive the bitcoin price up based on scarcity law. Please correct me if I'm wrong...
That is the 1 million BTC question.
While reward going down doe cut future supply and should be bullish to prices how much and how quickly is hard to gauge. Remember the existing 10 million coins will still be available for trade. Revenue in BTC will definitely be falling 50%. Prices may rise but they likely will rise less than the 100% necessary for an "even trade" and the rise is more likely to be felt in the long term.
A miner in the short term likely will see a significant drop in revenue. If block goes from 50 * $5 = $250 (today) to 25 * $7 = $175 after the cut the fact that prices will rise in the future may be of little comfort for the marginal miner.
Simple way to avoid that is to not be the marginal miner. Marginal miners get squeezed out and difficulty falls boosting revenue of those who "survived". Don't be the marginal miner.
However, FUD will shock the market. It is very likely that a bubble will form (like the one formed in May if I recall correctly). How high the bubble will go and
when it will burst is another question. Another interesting effect will be the velocity of price-decline after the burst. It may take months before the price comes
down to what it is Today ( if it ever stabilizes at this price, I expect that it will be higher ).
Another interesting effect is that certain people who have huge reserves in Bitcoin will see instant value increase in their assets. This will prevent them from selling,
since they'll be waiting for bubbles peak. This will decrease the number of Bitcoins available for trade, which in turn helps increasing overall market price of the
Bitcoin... Also, those who continue mining with GPU will expect revenue decrease and may stop mining ( if the bubble kicks in late ), which may reduce network difficulty
rate, unless it's componsated by miners who increase their processing power in preparation for the big-bang day.
I believe that many miners are already considering extreme processing power increase (2x minimum) to compensate for the expected revenue loss..