When you say cost, I assume that you are speaking about selling price
. Selling prices are generally set by the market. A seller wants to sell for the highest price
he can get, and that is based on the market competition more than any other factor.
If you are referring to build cost, keep in mind that cats have doubled up on some equipment
. That, no no means, is "the" reason, but it is a factor that increases build cost.
Now that you have a simple response, let the mono vs cat discussion begin...