I can almost guarantee that Kim Jong Un and the rest of the DPRK leadership do not think that it is 'worth a try' to get political brownie points by releasing some prisoners. This is not a new occurrence in the upper echelons; this sort of thing is routine, and it's sole function is to keep up the facade of a benevolent leader, from Kim to Kim to Kim. It has not swayed any voter opinions, it has not caused world leaders to rethink their negative positions.
(we already know the leadership in North Korea either does or doesn't care...)
So out of two variables, we know FOR A FACT that it is definitely either on or the other? Sorry, this was just funny. Nothing to do with anything.
The army (guards) are paid in rice, as are the rest of the lucky working population. The rest, who are not working (the majority/prisoners), get nothing, literally. This might not seem bad since you can say "oh, the government doesn't give me any food either", but when you consider that the entire country is on rations, and it's 'officially' illegal to buy and sell your own food, then it gets a bit difficult. It's only recently that a blind eye was turned form growing your own vegetables.
It was not a cost cutting exercise; the resources saved would be less than what it cost to send out the report of them doing it. It was propaganda.