Tiered SaaS plans w/per use individual service billings. These are just musings,

billing_musings.rb

class FreemiumPlan
  include Billing::Plan
  
  def cost
    0
  end
  
  def number_of_users
    5
  end
  
  def number_of_contacts
    20
  end
  
  def storage_space
    100.megabtyes
  end
  
  def initial_credit
    100
  end
end

class Costs
  include Billing::Cost
  
  def sms(attributes)
    case :region
    when :north_america
      0.01
    when :europe
      0.05
    end
  end
end

class Ability
  include CanCan::Ablity
  
  def initialize(user)
    can :create, Contact if account.contacts.count < plan.number_of_contacts
  end
   
  private
  def plan
    acccount.plan
  end
  
  def account
    user.account
  end
end

class SmsController < ApplicationController
  rescue_from Billing::NotEnoughCredit do
    render :text => "Your account does not have enough credit", :status => :payment_required
  end
  
  def create
    @sms = Sms.new params[:sms]
    bill! :sms, :region => @sms.region
  end
end