One thing overlooked here (or not explicitly called out) is talent pool. My last boss didn't want to use anything that he couldn't plug into Dice Or Similar and get dozens of hits for.

If it's not part of your core business/core competency, and you're just reinventing the wheel so you don't have to pay a vendor, you have to weigh against the cost of the off-the-shelf solution NOT just the cost of developing it yourself, but developing it, maintaining it, documenting it, replacing your "irreplaceable engineer" when s/he quits, etc.