That was not because of declining sales, it was just a case that displays are probably in greater supply and with time the part has become cheaper for apple to source / manufacturer.
It was clear with them keeping existing macbook pro's last year at normal price that they needed to keep the bottom line, but the costs of MBP Retina prohibited them from offering them at the bottom line pricing but keeping same profit margins.
Now that the part is cheaper they can finally start to lower the price differential and get rid of some of the older legacy models from the lineup.
Things haven't always gotten cheaper, mac pro's have fluctuated up and down with the newest 2013 edition being more expensive than any of the last few revisions.
The price of iPhone's too has pretty much increased slightly each year (unsubsidised pricing) forgetting the 5C which still is hardly cheap unsubsidised.
So really there is probably a mixture of factors that govern pricing, including parts / supplies and of course market economics. So the answer is really a combination of factors.