Ask HN: Software is cheaper to build now

With software, I have had this theory for a while that there is somewhat of a law where what was built one year ago or 10 years ago is always cheaper to build today. So this is what helps continue to drive innovation. Have there been any studies on the price of building software always getting cheaper?

I suppose a big exception to this rule would be when there are major players in a particular niche. For example, it would be more expensive to build a competitor to Google now than in 2000. But it still follows that the code it's cheaper to write now because there are so many more tools available to the coder than before.