There are no ongoing costs (according to a friend at AT&T)
From the Apple page...
All Apple Watch models feature Bluetooth and Wi-Fi. Apple Watch Series 3 (GPS + Cellular) also delivers cellular connectivity independently of your iPhone but must share the same carrier. The following companies are part of a growing number that support Apple Watch.
Apple Watch Series 3 (GPS) requires an iPhone 5s or later with iOS 11 or later. Apple Watch Series 3 (GPS + Cellular) requires an iPhone 6 or later with iOS 11 or later. Apple Watch and iPhone service provider must be the same. Cellular is not available with all service providers. Roaming is not available outside your carrier network coverage area. Wireless service plan required for cellular service. Contact your service provider for more details.
Apple Watch - Carriers
From the Apple page...
All Apple Watch models feature Bluetooth and Wi-Fi. Apple Watch Series 3 (GPS + Cellular) also delivers cellular connectivity independently of your iPhone but must share the same carrier. The following companies are part of a growing number that support Apple Watch.
Apple Watch Series 3 (GPS) requires an iPhone 5s or later with iOS 11 or later. Apple Watch Series 3 (GPS + Cellular) requires an iPhone 6 or later with iOS 11 or later. Apple Watch and iPhone service provider must be the same. Cellular is not available with all service providers. Roaming is not available outside your carrier network coverage area. Wireless service plan required for cellular service. Contact your service provider for more details.
Apple Watch - Carriers