Set it up like a guild treasury, centre costs 50 for 10 men and the lord sets the price to 40 and puts in 500 gold. When people recruit from the centre they pay 40 and the difference is drawn from the treasury. The treasury can't accumulate debt so only 50 men would be recruited at a subsidized rate. Likewise if the price is set higher, treasury accumulates gold and the lord can take gold out.
