Is there a benefit for paying 2000 gold to mattis for the key at last ligth in? (Act 2)

[Wabbajack]

Is there any benefit/unlock new things later on or nothing?

[Orion Invictus]

RP value, some characters approve of it.

[Raelic]

Not that I found. He's nice to you later anyways and doesn't mention the actual amount so haggle away.

[General Malaise]

I doubt it.

Not tried it, but I imagine you could even pick the lock that it opens. I always got it free.