An intranet-in-a-box product charges per user, every year, forever. A custom SharePoint intranet is a one-time build. Somewhere around 50 to 100 employees those two lines cross - and where you land on that line decides the whole question. Here is the formula, and the case where buying still wins.

Vendors selling dedicated document management systems rarely mention that the buyer already owns most of what's on the quote. The real comparison pits configuration you haven't done against licenses you don't need - and in a few specific situations, the dedicated platform still genuinely wins.