Replacing an oxygen sensor should take around 1 hour less and some shops charge $100.00 per hour while other shops charge $50.00 per hour.
Then there's the cost of the new oxygen sensor which ranges from $20.00 to $70.00 and shops can up the charge of the price of the part which is normal.
When I had an oxygen sensor replaced on my car it costed me $150.00 for parts and labor but it can depend on your car and how hard the oxygen sensor is to replace.