Hi @odelacruzc - In our 8.3 stack release, project monitors only worked with our bets hosted infrastructure. This would be what's going to be charged for. In 8.4, we are releasing support for Private Testing locations. Once you set up a private testing location, you'll be able to point your synthetic projects at that location and run them for no additional charge.
Until 8.4 comes out, feel free to run your tests on our managed infrastructure as you will not be charged for that test execution whilst the service is in beta.