How much of the statement of purpose should be focused on each of my academic background and my work experience?
The SoP should focus on what you can add to the programme that no one else can. In your case, it sounds like the work experience is really what sets you apart. The students who apply for PhDs will generally all have taken difficult classes and gotten good grades in them, so that's not something you should spend time on in your SoP. The only exception is if any class contained a project or small dissertation or research component where you did really well: this should be mentioned. Anything else will show up in the transcript. 

So yes, focus on the experience you got from working, and how that will make you an asset in an economics lab.