[altrus]

Writing robust cores is usually substantially harder than writing the equivalent software library. You usually have to design the logic to satisfy various different criteria (area, performance, or power), and, especially when implementing high speed design, take into account the targeted device.

Unfortunately, the number of people who are competent to design, test, and implement these cores is fairly small, and usually well funded. Oftentimes, these cores represent very non-trivial time savings, especially when it comes to characterizing performance and closing timing, so there is tremendous value in the marketing and selling of these cores.

On the other hand, sites like opencores are great for finding useful IP that people have contributed and may be adapted to your device.