If you're looking to monitor an Android device (version 4.0 or later), you can use the Copy9 app to hack and monitor the phone. The app has a paid version starting at $21.60 per month.
Copy9 comes with 18 different features, including GPS tracking and sound recording. The app tracks the hacked phone's information even when the phone's offline, and once it's connected to the internet, all tracked info is uploaded right into your account.
A downside of the app is that you need to have the target phone in your hand to install the spyware app onto it. When you hire a hacker, he or she can access the phone remotely, which, according to one Quora user, could cost between $500 and $5,000.