Once a book is printed, there are virtually no additional costs associated with selling it. Even if it was reprinted (it wasn’t; in fact, the publisher returned the copyright to my coauthor and me) the reprint costs are negligible. For math books the major cost used to be typesetting. That has disappeared and all mathematicians typeset their own papers and books, using T \epsilon X.
If I bought your book for $62.50 in 1991 and wanted to recoup my investment I’d need to charge $133 today.
Maybe no one wants it and it is worth $0. Maybe collectors want it and it is worth $10,000.
Maybe it is coincidence that your book is worth very near its inflated value today.
But it is still available new from the publisher for its original list price. And I reject the marginal economics argument that its “value” is only what someone somewhere is willing to pay! The fools! If only they knew!