I am seriously considering pulling the trigger on this!!! That is under 2 bucks per book....with free shipping!!
https://www.ebay.com/itm/Hardy-Boys-Casefiles-1-127-Complete-Set-High-Grade-Excellent-Condition/202248914783?hash=item2f16f9875f:g:UDQAAOSwGjZanEF3
That does look good. It would take a while to find them all separately.