pricing_model.rb

class PricingModel 
  def initialize(config)
    @configured_components = [
      :setup_fee,
      :unit_price,
      :quantity_interpolated_price,
      :area_interpolated_price,
      :sheet_price,
    ]

    @initial_fee          = config.fetch(:initial_fee)          or @configured_components.delete(:setup_fee)
    @quantity_checkpoints = config.fetch(:quantity_checkpoints) or @configured_components.delete(:quantity_interpolated_price)    
    @area_checkpoints     = config.fetch(:area_checkpoints)     or @configured_components.delete(:area_interpolated_price)
  end

  def price(price_case)
    @width  = price_case.fetch(:width)  if @configured_components.include?(:area_interpolated_price)
    # @height = price_case.fetch(.... )
    
    @configured_components.map{|c| self.send(c)}.sum
  end
  
  
  private
  
  def quantity_interpolated_price
    @width * @height * Interpolator.new(@area_checkpoints)#.x ...
  end
  
  def setup_fee
    @initial_fee + 0.2
  end
  
  # ...
end