I must say that it is one of those areas that we often undervalue our own work but overvalue the work of others. The number of times I have paid decent money ($7-$49+) for something and found it to be very underwhelming for the price. Like a 5page PDF for $7 which barely met the promises of the sales page and other people selling literally a collection of blog posts for $49. But I do think that all bloggers need to be able to sell a product and make money if they can back it up with the value.