Cost of motor plus %
Cost of engineers wages plus%
Cost of any consumables used during fitting plus %
Cost of travel
The % pays for the office and staff, accountants, insurance, training, trips to wholsalers, site surveys, tools and equipment, van servicing, warranty work, bad debts, taxes, phones, electric, work wear, safety equipment, holidays, sickness, and other stuff i have forgotten.
The age of the system makes no difference to the cost of replacment except what the motor may cost.