Is there any rule of thumb for the equivalency in cost between household knights and say crossbowmen? I mean, how many crossbowmen would a Westerosi lord be able to keep at his disposal with what a household knight costs him per annum? If you were a Westerosi, which would you choose, a household knight or a group of crossbowmen?

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.