The biggest challenge with self-treating is being sure it happens each month when scheduled. As with any preventative effort, it's easy to push it down the 'to do' list. Frankly, I'm not real keen on handling the chemicals.
We have professional pest control services that run about $200/month for our facility (approx 80,000 sf). This includes spraying inside and outside buildings, as well as maintaining numerous bait stations. They are always helpful if I have a question or concern.
I'd be curious to hear what others are paying?