Hello Martin,
Now I am sure there will be others that will wade into this, which may give you some additional information. On what you just wrote I am going to give you a right off the top of my head and say it depends on a lot of elements:
1. What region you are in, and if it is mostly warm or cold and what are the main critters there are problems with?
2. Prevention is the best approach rather than waiting and then trying to control it.
3. I would look at what it is costing you for the month and then see about having a firm which gives you a package deal to come at certain times during the year. All firms are not equal. When you get a firm, Like the ones that I would contract there was a clause that in between their coming out to service if there were bugs that came back and did the whole thing for free. So, when they came they did a thorough job as there was a loss of money for them to come back and do free.
How many units do you have on your property is also another factor. Now another thing that I did before people first move in, I did a spray of their unit empty unit. This was incase they brought some critters with them from where they were before. These type of sprays the maintenance folks did.
Now, it does not take a rocket scientist to do this and so it would not hurt to get one of your maintenance people certified if possible and then you can cut the other bill all together.
This is a fast once over the world.