Actually, thanks to Quark and InDesign, you can convert content from print to web to PDF documents with the click of a mouse--assuming there was some forethought toward this end from the beginning. As borisb says, however, for previously published books, renegotiating rights and permissions can take the better part of a year for one book!

As for the cost, the most expensive part of old-school publishing used to be the paper. Now, in an increasingly paperless world, it's the cost of hiring seasoned writers and paying them enough to get interesting and accurate content, then hiring a skilled designer to make it look good in several different media, then a copy editor to polish it all. Those costs are fixed relatively, no matter what medium you choose to distribute your content.