Depends on the cassette. Give examples. If we're talking mainstream metal albums on tape, then not even worth the effort trying to sell them for the most part, you could have an entire lot of 100 and make maybe $20 at best. A few exceptions, Rigor Mortis' major label debut cassette can fetch some money just because of the size of cult around the band for example. If we're talking demo tapes from the mid 90s and later from underground bands no one cares about, then maybe $5-$10 if it looks cool. If we're talking demo tapes from the mid 80s, then anywhere from $50 to hundreds is possible, especially if you wait.