You're right, you will have to pay the airport tax at the airport... the domestic fee, a few dollars, for intra-Peru.
That 12000 was return? Thought it was one way, my error. Maybe worth thinking about...
but then again at 2 cents/km 12000 km equals $240. Base Plus fare LIM-ARQ-LIM is $197 (Base fare $79 jeje). But if you want to burn kms, it is a nice sale (normally 18000).
Thanks for highlighting this.