what does it cost on your karma scale if someone ruins your life . .

none

for it to propel, it needs action. karma means action. if they did that something untoward, it will be their own karma, not yours, like a pen would not move without the elements like air, the eyes that can see the pen, the manufacturer, trader of the pen, the farmer that supplied the food for the ones who made and sold the pen, sunshine, soil, air that nourished their food, etc., the space that the pen occupies - endless variables to point out, as we are all interconnected

in regards to why you might have been entangled with their not-so-good karma, it's because there's such a thing as individual and collective karma

individual is what you've sown yourself, collective would be the karma of people in your circle, such as family, loved ones, connected in the past by actions done by both you and them, and in association with them, as it's true that if you know that something wrong has been done and didn't do a thing about, both individual and collective will be incurred

again, none on yours, but keeping a pure, forgiving heart can help dissolve the bad seed

I wish I could have laid it out short and simple, but the the subject of interconnection itself is infinitely immesurable