At the factory is a good bet.

If you choose any engine in the suggested range it's going to weigh about what any other brand of engine weighs. And all the radio/servo stuff isn't going to deviate much by brand. So all the stuff you have brand options on will wind up within the same total ounces and the ARF designers really ought to have that figure in their advertised weight. What's left?

What the factory used that's underneath all that pretty covering.

One ~600 sq-in wing for an ARF I built a couple months back weighed almost 2 lbs with two servo's in it.
One ~600 sq-in wing for a different ARF I just built weighs just over a pound with two servo's in it. Amazing. This lighter wing is for a CAP that I throw around pretty hard. It's strong as ......

The two wings came out of different factories.