Realistically, when it happens a bunch of people will shut down their rigs b/c of electricity costs, then the difficulty will go down. So although the blocks will be only 25btc it will be easier to get them. Similar to what happened when the $ value of the btc crashed.
Well, there're about 7200 new BTC mined each day. Let's assume that about half of them is to be sold, which makes constant supply pressure on market. As a result, the bitcoin price may get doubled. 25, 50 are just nominals which does not correspond to market value. Difficulty may remain on the same level.