Contact your local monitoring station. Most Brinks systems are now covered by ADT monitoring stations because of the merger between the two companies.
Ask to speak to a manager, since a regular operator won't have the authority to negotiate your rates as much as a manager will.
Explain the type of system you have and that you want monthly monitoring. From there, you can work out the monthly rates and what services will be provided. Be firm in how much you can afford to pay to avoid going over budget. Find out if you have to sign a contract, and whether the contract requires that you have the service for a set number of months.
Get the agreement in writing to verify that you agree with all the terms and conditions prior to the start of your monitoring service.