Although you can buy a DSL or cable modem that is integrated with a Wi-Fi router as a single unit, this is not that common and has a couple of drawbacks—namely a probable higher cost and less ease of upgrading the modem.
It's really dangerous mentioning dollars-and-cents figures in a book for several reasons. The cost of computer-related devices is constantly changing, mostly going down over time as specific technologies become more mainstream. By the time you read this book, it's a safe bet that it will cost less to create a Wi-Fi network than it cost as I was writing the book. (And it was inexpensive enough then.)
It is also true that there are many different possible network configurations, as explained earlier ...