I wouldn't pay the dealer to install splash guards. Can't you put them on, yourself? If you really want to pay the dealer then they'd probably gouge you for $150-300, I'd guess.
We had the aluminum trim start to discolor, as well. I washed the car really well and applied some wax to the trim. It seems to have helped make it look a little better and it's not getting worse... So I'd definitely recommend putting some wax on there to at least stop the aging.
2018 Infiniti QX30 Premium AWD - Majestic White
2019 Volvo XC60 T6 AWD Inscription - Onyx Black Metallic
2018 Honda Gold Wing Tour - Pearl White