If all CDs were available for $5.99 including shipping (or download), I'd buy a heck of a lot more CDs. In fact I already do as I have been using yourmusic to fill my back catalog, replace LPs in a more artist friendly way than buying used.
This isn't realistic. You are forgetting where most of the money goes: marketing. It costs hundreds of thousands, even millions, of dollars to launch and promote an artist.
You listen to a CD over and over again and pay $15 for it. You read a book once and pay at least that. But no one is up in arms over book pricing. And what about DVDs? You pay anywhere between $10 and $20 for a DVD and maybe watch it three times.
When it comes to the consumption of content, CDs are still dirt cheap compared to other stuff - when one considers how many times one "uses" the content. Sure, labels could sell CDs for $5.99, but there would be no videos, no tour support for artists, no advertising of the new release, no website, no radio servicing, etc.
As someone who ran an indie label for years, I can tell you that CD manufacturing is very affordable. It's the advertising and promotion that eats you alive. Running ads, even in smaller "zines", still ends up costing thousands of dollars. Try advertising in Rolling Stone.
Do some simple math. Let's say that it costs $1 to manufacture each CD and you manufacture 2,000 dics. You then spend $10,000 on marketing. You've now spent $12,000 on the release - or $6 per disc. You sell to a distributor for $8. They sell to a store for $10. The store sells for $14.99. At $8 per disc, you need to sell 1,500 copies of the album before you even make a penny. And in the indie world, 1,500 copies is quite an accomplishment.
Now imagine the numbers on a major label scale. Your marketing budget is measured in the hundreds of thousands of dollars. You have to advance the artist a large sum of money. Etc. How many albums do you need to sell to break even? 50,000? 100,000? Who knows. The point is, marketing music is very costly - and given all those costs, CDs are still pretty cheap.