What do you guys think?

https://www.amazon.com/APEXi-410-A001-SMART-Accel-Controller/dp/B007ZDR8FA

Will buying a throttle controller give me better gas mileage if I drive in ECO mode??

Right now I average about
22 MPG ... get roughly 325miles per tank

I fill-up for about 70-75$ CAD a week. (Roughly $60 USD)

Is it worth spending $318 CAD?
Will the savings in gas mileage pay this thing off?

I'm not interested in performance since it's winter in Canada; driving fast is a no go ):

 

No, it simply changes how sensitive the throttle is. You'll achieve the same result by driving like your grandmother is in your backseat wearing her Sunday finest while having an open pot of gumbo on her lap. I would also maybe suggest Snow mode as it dumbs down the throttle a bit as well.

There isn't much you can do to improve MPG on these cars besides your driving style.