Most of these companies simply copy other companies' tapes. This became apparent when several tapes purchased from Company A contained spoken advertisements for Company B, etc. These tapes are all in the Public Domain and are not copyrighted. The material cost for bulk cassettes is less than 50 cents. Even when including the cost of vinyl albums and labels the entire per-tape material cost is below a dollar.
Many companies have shipping charges well above their actual shipping cost.
Audio quality on average is very poor. This is primarily a reflection upon the original recording method. Companies which sell CD's and claim that the audio quality has been vastly improved are misleading their clients. In almost every case the CD is of no better audio quality than the cassette tape because of limitations regarding the quality of the original recording. Bob Y. runs this great site which has many RealAudio recordings.