I'd say that the larger budgets are a combination of factors;
1.) The value of money is diminishing as things are becoming more costly.
2.) Average Joe wants to see big explosions and cool special effects.
The bad thing is that the interpretations of profitability with film are changing from what they were previously. According to an article a few months ago about the possibility of an Incredible Hulk sequel, film studios now want to see at least a 25% increase over the break even point before they consider a film a profit. If Terminator's budget (I have seen several different figures) it would have to make a minimum of 250 million to be considered a success.