My dealer installed cost was $850 and they did it before we picked it up new. I bet you could do it yourself for less. I am not sure how big of a deal it might be to wire. Mine was pre-wired. So does your unit have 50 amp service?
Here in Missouri I consider it absolutely necessary in parts of June - early September at times. There is an entire thread on this subject. I guess it depends on what one wants or can tolerate. Once it gets above 90 and humid a single unit can struggle to maintain even a 20 degree differential during the day. We normally set the AC at about 75. It also really helps in cooling the entire unit off much faster when we arrive and setup. It can take hours for the single unit to cool it down initially since the entire inside of the unit is hot (i.e. walls, ceiling, stuff - basically a mobile solar-heated oven on the road).
I sleep the best in the 312 with my wife's snoring than at home. The 2nd unit completely drowns her out. I will always run the fan if not the AC at night.
2016 Coachmen Freedom Express Liberty Edition 312BHDS (37')
2014 F150 Lariat 3.5 V6 Eco Screw max tow - Century hard cap - Husky Centerline TS WDH