The common DI is a complete joke because it works off a primitive internal gyro.
Any half decent plane will have a DI which a) uses a much more stable remotely mounted heading gyro (cost £5000 or so) and b) the heading is continually corrected with a heading signal from a fluxgate magnetometer (basically a solid state compass) mounted in a wingtip (cost another £5000 or so).
This technology, called a "slaved" DI or HSI, is decades old but works very well.
A glass panel will need the above remotely mounted equipment too. Today, one can avoid the remote gyro with a fibre optic reference system, but it isn't any cheaper.