Is this phrase commonly used?

 

No, hardly ever!

We would say thanks a million

 

I'd say it's relatively common in AE. There's also Thanks a lot/a bunch/a million/etc.

 

Thanks a ton for your help!

 

It's not common in British English.

 

Ugh? Can we define common?

GF..

 

I'm from Atlanta, GA, where a neutral, cosmopolitan form of AE is spoken, and thanks a ton/lot sound completely natural and common.

Thanks a million also sounds natural, but is less common around here.