I play the free rock balls exclusively but you say that’s not an option for you. So, if you have a mobile device and download the WGT App you can watch a maximum of 75 videos per day and earn 150 credits. That’s 1050 credits per week and other players say that’s enough to buy decent balls to earn some credits in RGs.
I never could get the surveys working but I know some players had success earning credits that way too. I think there’s an issue with the surveys now but I’m sure other players will assist and shed some light for you regarding them here.