johnr j hits it right on the head. A good and comprehensive service proposal should take into account all the factors he mentions and possibly more. At a minimum a typical proposal will include a basic PM rate ( flat rate ) and a complete explanation of what is included.
Along with that, the proposal should include a quoted labor rate for remedial and emergency repairs as well as a repair parts pricing rate. It should also be clear as to what surcharges and or additional fees might apply ( fuel, shop supplies , environmental fee's ). Other things to consider might be the proximity to your facilities and parts availability. You might also consider support, Will your salesperson/ CSR be there to act on your behalf if there are issues or problems.