nat you have said it will take a long time to go round and price the properties have you considered how long it will take to clean all these properties , do you have the employees and vehicles to cover this extra work and if you do how can you send your employees out with a list of what they should get through if you havnt viewed them and got a rough estimate of the time each one will take including travelling time between jobs.
in my opinion you must veiw each job or on this size of contract you could lose serious money.