Generally speaking in an EU context, I believe there's enough prior court rulings in EU countries to suggest that an airline shouldn't bond you for any more than the training has cost. Airlines can, of course, inflate the actual cost somewhat on paper. "Seems an arbitrary figure", therefore, could have been carefully constructed as a result. How long they can bond you for and if/when the bond reduces over time would be more contentious issues.
I don't think there's anything definitive, I'm sure it would vary from country to country within the EU, and as pointed out they have more than one way of skinning you in any case. The airline line is that the reason bonds exist is that people like you (no offense) took what they could and ran off thumbing their noses at the airline, if you don't like it don't sign it and don't work there, and if you did sign it, suck it up and live up to your legal responsibilities.
It's like the old joke "Doctor, my arm hurts when I bend it this way". "Well, don't bend it that way from now on".