Hi,
When the iPhone is at home or on a WiFi Network then iMessages are free.
SMS will still cost as per your carrier service.
On the Mac iMessages is free (you are connected to the network).
SMS Forwarding is sent via your iPhone and we are back to the carrier service again.
IF the iPhone is only connecting via a carrier service then data changes apply for iMessages as has been stated.
To be clear.
iMessages is an Account on a Mac version of Messages
It send iMessages and the service also tends to be referred to as the iMessages service.
On an iPhone or other iOS device the App is called Messages and it can send iMessages or if to an non Apple phone (or unregistered for iMessages iPhone) it will send SMS messages.
9:06 pm Tuesday; May 26, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad