Most of my development experience is creating in-house solutions, but have had my fair share of "buy-ins" on both sides.

My instincts say that for any internal processes or business-specific requirements, write your own.

For any "common" functions, try to buy something in that best meets your needs. If you can't find any, make your own.

Of course, when buying anything, it's much easier to justify it if you're getting the source-code as well. I was able to sell an internal component I wrote to another company, almost single-handedly though a friend, and that was pretty straightforward as my company was willing to pass on the source code as well, (not sure if without the sale would've happened anyway, but it's a possibility).

Paul