The aspirational punctuator strikes again! I would like to punctuate this so that it reads “Good my lord –“ Because there’s something about this exchange that makes it feel like Rosencrantz would like to say some more to Hamlet, to engage him a bit before they part. It feels like that in Hamlet’s response to him too – a sort of “Yeah, yeah, get out of here” quality.
With a period at the end of this line, it could be read as an agreement. Like a “yes sir” or “As you wish” but that’s not nearly as interesting.