Yea I think the budget thing is more of a coincidence, or rather a secondary cause. Or maybe even an effect really: the films got better and better and the studios gave them more and more money. I think the biggest thing is the actors grew up and really molded with the characters. Not to mention a lot of these were no-name actors coming in, and here they've pretty much honed their craft in this franchise. They became more mature and honed their art skills. That and coupled with the fact that I think the books just lent to better and better movies. Another example of just becoming better at your job (the writer of the books). Plus they started having more and more substance and depth and maturity to the stories. I think this contributed to it becoming better.

But yes, this new one could be the worst hunk of garbage since Battlefield Earth. The name alone, however, will make HUGE amounts of money though.