An average (~worthless) book is something like a pulp romance novel, political book by a sitting politician, self-help guide, .... These are printed in the millions and used copies can typically be found for $1–$5 + shipping costs. That’s not the same kind of books primarily shown/described here.
Any scholarly person who loves old hardback books and spends a few decades collecting ones they personally want or need is going to end up with some worthless books, a large number that sell for $10–50 each, and a few that are worth hundreds each. It’s just inevitable, unless they go out of their way to only collect junk.
If I had to guess I’d put the price of the old man’s collection in the $10k–$30k range. But it’s plausible it could be more, if he collected anything rare.
Any scholarly person who loves old hardback books and spends a few decades collecting ones they personally want or need is going to end up with some worthless books, a large number that sell for $10–50 each, and a few that are worth hundreds each. It’s just inevitable, unless they go out of their way to only collect junk.
If I had to guess I’d put the price of the old man’s collection in the $10k–$30k range. But it’s plausible it could be more, if he collected anything rare.