Using a spam blocker service like SpamStopsHere is all about economies of scale. If you wanted to call your mother on the telephone, you wouldn't bury your own cable all the way to your mother's house, or hire your own repair crew to manage the cable. Instead, you would find someone that is already doing this, that specializes in this, and that is doing it on such a big scale that they can manage the service for thousands of people for not much more work than managing it for hundreds of people.
With these principles in mind, not only will you pay less than doing it yourself, but you will get better service than you would by doing it yourself.
This quickly rules out spam blocker appliances, as they require your own upkeep. Additionally, you will pay more for one appliance that you will have to manage, which will be far less productive than the huge cluster of redundant servers that you would get with a spam blocker service.
A spam blocker software that you install on your email server or deskto has the same weaknesses. You would need to manage the installation, and upgrades, and also the spam wouldn't really be blocked. Blocking actually requires that the email is blocked. All spam blocker programs don't realy block email because you've already accepted the email. They just delete it after you failed to block it. Also, you've let the potentially dangerous and resource consuming email onto your network and onto your computer already.
Spam blocker software that your own IT department makes from open source products and is completely managed by your IT department is going to consistently lose you money. Unless you pay your IT department pennies per hour, it's just not cost effective to pay someone an hourly rate to spend up to 10 hours or more managing your local spam blocker, when you could be paying pennies per user to a premium spam blocker service. Especially when your IT person likely isn't even going to do near the job as the professional companies that specialize in blocking spam with proprietary products. I don't care how good your IT person thinks he is at blocking spam, he can't beat the pros. You'll get many false positives and tons of spam if you really think that you can trust your IT team to do a better job.
The truth is, that if you want a positive return on investment (ROI), you need to use a premium spam blocker service.