SeaOfStars asked in yesterday's post whether a knight killer crossbow should weigh more. I dug into my notes taken from GURPS and historical sources and the answer is sort of.
The crossbow would be bulkier but not that much. Probably 12 pounds. If you want further balance you could say that you need a minimum strength of 12 or suffer a -4 penalty to hit. This accounts for the increased recoil.
The cranequin used to cock the crossbow weighs 9 pounds. But it is also detachable. You could argue that a built in stirrup and windlass is accounted in the extra weight. As for price much of the crossbow is still wood and still a relatively simple mechanical device. If you using a feudal society where the knights are effectively law enforcers the Knight Killer could viewed as an illegal weapon. Especially with it's name.
However to be honest saying that it double the price and double the weight is not going to have all that much of an effect in the abstract combat system of D&D. It is a single shot, highly accurate, high damage weapon. If the party relies on using this as their main attack and the first volley misses they are in big trouble.
Finally this was developed when the shoe was on the other foot. The party were the city guards dealing with adventurers. When the next campaign came around they had to deal with the consequences of their own innovation.
In of itself the Knight-Killer looks like a freebie handed to the players. However it makes sense when placed in the context of the campaign I ran.