I read that in the 1790s, Friedrich Wilhelm II ordered the banning of the cuirass; whether or not that was due to financial reasons or some twisted 18th century morality, I'm not sure.
It's worth remembering the generally poor state of Prussia during the Napoleonic Wars, I'm not sure whether or not it was cost-related but it really wouldn't surprise me.
Makes me really look forward to my Prussian playthrough, though. Ramp the difficulty and crank the Hohenfriedberger!
That's true, their tactics and weapons were as old as sixty years out of date and given what it would cost to run the kind of military the Prussians wanted, they may indeed have had to throw them out.
I, too, am looking to restore the kingdom of my ancestors to it's former glory. Assuming they are allied to the French, care of the treaty of Tilsit, I think it might be something to weaken the Russians, not invade, just taunt them to cross our borders, and wage a strong defensive campaign.