I just went through putting in an alarm and I can give a few tips.
I would look into a local security company. You can find ones that have much cheaper monitoring than the big names like ADT/Brinks/etc.
Also the alarm system world is very closed doors still. Somehow the industry has kept info private and they pride themselves on that but it makes it very tough for motivated people to go the DIY route. (it is also so they can charge a lot for fairly simple equipment, the markup is insane) It really bugs me as a lot of the alarm system technology is very dated and simplistic compared to what they could be making for the same price today, but in any event there is no other option.
The main parts of a system are the monitoring service and the system in your house. Monitoring services will be a monthly cost (some are prepaid for a set amount of time) and many times will only work with certain systems. Also a lot of companies require you to have them install it.
There are a few places on the internet that sell monitoring service and it is cheap compared to the big names as well. One I have heard of:
http://www.homesecuritystore.com/
Overall I would look into some local dealers and see what they can offer. Be aware that many are pretty old school as is a lot of the equipment compared to what we are used to from the digital age in the last few years.