In some ways, now is the "right time to buy" CDs - at least it appears to be so in the UK. On the Amazon UK site, I rarely need to spend more than £4.99 ($8) for a brand new CD from popular, big name artists. For example, Elbow's back catalogue is available for £3.48 each. Joni Mitchell's Blue has been available for under £3, but today I checked it has gone up to over £5.
I just use the Amazon wish list and occasionally check how the prices are moving, and also compare how the marketplace prices compare to Amazon's own. Sooner or later, the price will drop right down if you're prepared to wait a few months.
This doesn't apply to rarities or most deluxe re-issues. Those may disappear altogether, or go up. There was one though that I held out for for a long time: an out of print old reggae album (yes, really). For ages is was available used in the marketplace at over £100. Then one day, it was there for around £5 (used, as new condition). So I bought it immediately.