Prices vary and what is being asked is not necessarily what an out-of-print cast album is "worth." The simple answer it is worth what you are willing to pay for it.
Polydor's original cast CD of YOU'RE A GOOD MAN CHARLIE BROWN was deleted very quickly and was sold for as much as $125. Now Decca has reissued it and the polydor release is a $10 special.
Many of the deleted Broadway Angel releases were priced at $35 - $50 each in some stores...but now most of therare ones have been reisuued by DRG including FLAHOOLEY, THREE WISHES FOR JAMIE and TOP BANANA. (So, can LITTLE MARY SUNSHINE be far behind?)
Still WHOOP UP and KWAMINA remain among the most sought CD's amongs collectors of flops.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com