It's a curious observation that an NDB must be a very cheap thing to run (could even be solar powered), while the receiving bit, the ADF, is relatively expensive.
The typical ADF, even brand new, is a 1970s design. I've just seen the innards of some in an avionics shop. There is no reason why it should cost £3000. One could also implement it by signal processing and measuring reception delays, i.e. without the present directional aerial system.
A VOR transmitter in comparison must be hugely expensive to install and maintain.
So an NDB ought to be a reasonable navigation fallback choice for remote locations. Inland, with no thunderstorms about, they are pretty accurate.