I recommend that you buy RS Means Lanscape Estimating book and learn how to use it. Not sure if they cover mowing, but they do cover plantings, seeding, site work, mulching, etc. Good reference and good tool to use as a check on your costs of servicing commercial accounts.
Also, if you are insured and pay worker's comp, etc. make sure to emphasize that and provide the limits of your coverage to the account manager. Commercial customers really appreciate that kind of thing because it shows that you are on the ball and have taken the necessary business steps to service that kind of account.
Above all, be as detailed as possible in your estimate. I would not reveal the hourly rate to them, rather bid the job as a whole. However, that could depend a bit on the type of job. Best thing is to make sure your costs are covered, don't worry about what the other guy is bidding!