I don't think we can make this assumption. Google may be selling these devices for 300/350 but we don't know how much Google pays LG.
That's just my best guess, based on the prestige of making the Nexus phone. I think Google could get someone to make it for a break even cost. But when you have LG execs saying they are trying to work with Google to get more of these phones out -- well if it is just manufacturing capacity issue they don't really need to work that out with Google. It kinda implies to me that if Google is willing to give LG better terms then LG can find a way to produce more phones.