Under current environment, it would be difficult to find outside takers who would pay enough money to build these reactors. They will get bombed or face other dire situations before they see their sales proceed coming in. Internally, they do not have enough money for this project, previous cash payment from S. Korea not withstanding.
I consider it wishful thinking to assume it is assured. It is not remotely assured, it isn't even likely right now. Bush is pulling political teeth to get something done even about Saddam, who can scarcely hurt anybody and is a tyrant so egregious his overthrow is obviously just any time it happens.
The leader of the south just won an election on an appeasement platform. China and Russia are flaking for North Korea, the former is sending nuclear supplies. Both China and Russia are consciously furthering, rather than restraining, gradual nuclear proliferation to rogue states.
As for how much they will be paid for nukes, my guess is the sky is the limit. Their current capacity will be a bomb every few months, but as they reinvest proceeds in more reactors that rate will snowball upward. Of course the bombs themselves will likely acquire US or Israeli final addresses.
Also, I think you underestimate what tyrants can easily do in diverting resources to one end. NK has the aggregate wealth to make reactors, obviously. It is perfectly willing to impoverish millions, work them as slaves, feed only the compliant, etc, to see its pet projects done.
It is not like they can't do it unless it is "economically viable" as an investment - practically nothing the country does is "economically viable", that is why the people eat grass. They are not going to be stopped by lack of wherewithall or lack of outside aid (not with China still helping them, etc).
They are not going to collapse internally and just go away before anything happens. Responsible powers led by us are either going to do something about it directly and forcefully, or the whole problem will indeed go right on blowing up.