I don’t think there is a rule of thumb. It’s more about finding a synergistic balance, and I’m sure that distribution of cost varies as one goes up the range towards higher end gear (or down towards less costly gear).
FWIW: I have a Clearaudio Innovation turntable ($15k), two Clearaudio Universal 12" arms ($6700 each) and two Lyra Atlas Lambda cartridges ($14k each). So my cartridges are double the cost of my tonearms. It’s an excellent balance of gear for this particular case, but I wouldn’t generalize on it at all. One also has to take the phono-stage into account. That matters as much.