Paying 100 points to give a 300 point monster doubled durability with a 4++ "ward" save and some cool bonus abilities is beyond a steal. I just don't see how the point system takes into consideration the multiplicative effects some of the rider options for mounts give.
then play it at 300 extra points. Seriously that's what playtesting is for, I can get perfect estimates on a model's value without special rules, but in some cases there will be no mathematical way to calculate the value of a special rule and as such you just have to place a value on it and test until it feels right. When I was playtesting my fanbook back in the day I toyed around with both the rules for the special rules as well as their respective costs. it took many MANY games to round out the numbers.

So anyone who wants to test the system at Dlair I'd love the help, but nothings perfect yet, this is the missing skeleton to what Age of Sigmar can be. I hope to flesh it out and make it into a fantastic tactical game.