They are also a lot of them about.
They've just about all disappeared in the US. Even VOR approaches are disappearing (the one at Stockton, of fond memory from training and currency practice, was just removed). They only remain, as "NDB or GPS", where there is so little traffic that it's not worth resurveying a proper GPS/LPV approach (like KSIY, the world's most improbable NDB approach).
I'm also under the impression that the operational costs of an NDB approach is low.
Hard to see how anything could be cheaper than GPS.