Sometimes CDs are cheap because of a low advertising overhead or a high production run, but at other times they're cheap for a very simple reason: because the people who made the CDs found a sneaky way to avoid licensing fees. Hence "weasel compilations," for lack of a better phrase.
When I started DJ'ing retro nights I fell prey to several of these CDs before I finally learned my lesson. They always START OUT with three or four of the classic recordings that you're expecting to hear, but then they slip in the FAKE songs.
How are they fake? They either use a live performance by a few original band members -- always recorded at some highschool dance or small-town festival, with stadium reverb added for odd effect -- or they get the elderly, coke-addled lead singer to come into the studio and record a new version with karaoke session musicians. These ones are the worst, because the instrumentation is terrible Casio keyboard stuff and the production is bottom of the barrel. Even if you don't know the song, you know it's a fake version because no record company would EVER release it as a single.
Hence the cheapness of the CDs. By only including a few of the classic songs -- which cost a bundle to license -- they then find the surviving members who own the music themselves, convince them to do a budget recording session or a live soundboard recording, and then mix those "updated" versions into the bulk of the CD...without ever revealing this in the song titles. If you're lucky they'll include a weasel notice on the back, in tiny letters, somewhere down near the corner:
"To achieve the best sound quality possible, some of these songs have been updated or re-recorded with one or more of the original band members."YOU JERKS! It's no wonder we're cynical nowadays, and also no wonder that most of us probably have two or three of these compilations in our own collections, in which Marvin Gaye or "(Two-Fifths of) The Temptations" sleepwalk through their hit song in the backroom of some Legion building in Iowa, cheered on by the one man drunk enough to have a good time.
My point? Just a cautionary tale, that's all. Look for that weaselly notification before you buy a box-set of classic songs from a bargain bin...and even so, expect to get ripped off.
PS: Strangely enough, if the CD company is from Eastern Europe or Russia, you'll probably get all the classic songs intact, albeit with humorous spelling-mistakes in the titles. THEY get around licensing laws because nobody can untangle the international copyright issues in order to actually prosecute (that is, if the company's address is real in the first place).