It's not THAT hard to get (hell I even think it's a little too easy.) If you captured Corrith (spelling?) and manage to secured trade right with enough factions and build up some infastructure then you should have enough cash to build this thing in about 10 years plus or minus a little. I've done it in one of my GSC campaign, took only about 10 years to do so (but I got lucky as at that time as the Romans leaves me alone.

)
Think of it as a house rule.

And beside, trust me, they're worth it. Just one of these, if use correctly, can turn the tide of the battle. I won 3 battle against Macedon thanks to this guys (had nothing else except plain old levy hoplite.)