Crossbowmen are far, far, cheaper than household knights – the whole point of a crossbow is that you can train someone to use one much faster and more cheaply than you can a longbowman.
The issue with crossbowmen, though, is that they’re mostly ranged infantry (although they would carry personal weapons) whose weapons are slow to load and reload. If they come under hand-to-hand assault by either infantry or cavalry, they’re in for a really bad time.
Knights are more expensive – training, arms, armor, horses, tack, and other equipment add up – but what you get for your money is both the offensive shock tactic of the knightly charge and the defensive value of the dismounted man-at-arms. So high cost, high value.
