I'm not sure that your example matches how people buy songs though.
If you go to the bakery and buy, for example, whole grain bread - you can go somewhere else and buy very similar whole grain bread for the same price (or cheaper) for almost the same product (minor differences in quality/taste).
For songs, if you want Song X by Artist Y, no other label sells a product that is the same. No amount of fudging makes them the same. As a result, there really is no competition. Everyone I know (biased sample, etc etc) seems to say "I want song X, let me go buy it" rather than "I want to buy some music, hmmmm, what shall I buy". YMMV.
As a result, I would argue that older songs will probably be even more expensive. If you actually go looking for some song from some obscure 60s band, odds are you really want that song from that band, and are probably less price sensitive as a result.
Thoughts?