Just for comparison purposes a unit of Empire Knights would cost 135pts using WMA against the rulebook cost of 110pts, whilst Chaos Knights would be 155pts opposed to the 200pts in the rulebook, monsters come out roughly about the same cost.
It's not quite 135 points. The cavalry you're referring to also has the extra rule "knights" which gives a +1attack in first rnd of combat, regardless of whether they charged. Only applies if facing the front..... Basically it's the same as the Cold One Knights bite rule.
For the 155 point cavalry i assume you're looking at the Brother Knights in the Teutonic Order army list. I may be wrong, but i don't think those lists are as black and white as they seem. Certain troops may be cheaper to encourage the wargamer to take a list close to historical accuracy. You also have to take into account their opposition, the Polish-Lithuanian Union has extra rules for nobles and leaders (for no cost) On that table the results for 1 & 2 are a disadvantage (but not too bad), 3 is nothing, 4 is okay, but 5 & 6 are brilliant (+1 command and a reroll for a failed cmd) That army also has cannons etc. I think they've reduced them to 155 to give that army a chance of winning.
There isn't really a formula for the stats in warmaster (unfortunately) but roughly it seems to be +25 for an extra Attack or an extra Hit, and +15 for an extra Armour. Obviously the reverse effect for reductions of stats. It doesn't quite work out, using this method Orc Warriors are actually undercosted, but i think the Orc army has a hard time as it is.

In warmaster ancients they have many cavalry units that are 3,3,5+ costing 110 points. Seems to be about right. For the chaos knights it's hard to judge, personally i'd say 155 is a bit too cheap. I'd have them at around 175-180 (+1 attack=25, +1 hit=25, +1 armour=15) Using the above method Chaos Warriors cost 135, and hvy cav 3/3/4+ costs either 125-130
Using this method i was thinking of adding rules for units that aren't in the lists. Such as swordmasters, phoenix guard and executioners. I didn't want to get bogged down with extra rules, i just wanted basic stat lines to reflect their abilities (ward save of the PG is all in their armour save. The swordmasters don't have shields like spearmen but they can deflect blows, hence 5+ armour) It's not a perfect equation, but not bad.

Swordmasters ...4 / 3 / 5+ ....cost 85
Phoenix Guard ... 3 / 3 / 4+ ....cost 85
Executioners (same as swordmasters)
EDIT... not sure 4 attacks is enough for swordmasters, those stats might be good for empire greatswords...... maybe this instead;
Swordmasters .....5 / 3 / 5+ ....cost 110
I gave this discussion its own spot, as it was not on "Monsters" anymore, but well worth keeping !Lex