If something cost x, what is the cost of something 3.5 times lower cost?
Pretend the cost is $20, so x=20.
Divide both sides by the proportion it will lowered. If it was 2 times lower cost, divide both sides by 2.
That's the format you should follow using whatever value you would like. The important part isn't the $10, as it will work for any value you choose.