Is it 'nonsense' or 'a nonsense'? 'Good value' or 'a good value'?

[CrazyOldCatLady]

As in 'that's a nonsense', or 'that's a good value' (in a shopping sense, not a moral one), as a complete sentence.

I keep seeing both of them and they make me shudder but are they actually wrong?

A good value def no. But you can say something makes a nonsense of something. Haven't heard it in any other context though.