The classes and prices are based on how "nice" the plot is as decided by SE. For instance, a house in Mist that is near the shoreline will have a higher price than the ones without a beachfront view. Just like real life. If you have done the housing quest for each zone, you can check the current prices and availability at each city's main aetheryte by choosing Residential District>Go to Specific Ward. Or as previously mentioned, the same list will appear if you enter a housing ward.