There are always exceptions. 37Signals is a known brand. If you're not known, it's hard to charge a lot for a pdf.
"There are always exceptions. XXX. If you're not XXX, it's hard to charge a lot for an e-book."
"There are always exceptions. I'm the world expert on home tarantula care. If you're not the world expert on home tarantula care, it's hard to charge a lot for an e-book."
"There are always exceptions. I'm a talented marketer. If you're not a talented marketer, it's hard to charge a lot for an e-book."
"There are always exceptions. I'm a former HFT project team lead writing about trading strategies. If you're not selling a book that can potentially make people thousands of dollars, it's hard to charge money for an e-book."