Cost to my Plan for adding prem. channels

I wanted to add a couple premium channels to my plan, but instead of it going up the amount listed for them , my bill goes around $100.00 dollars more a month.

What is the deal,  I would think it would go up the say $20.00 but not over a $100.00

You make it really hard to add any services if you are going to be that harsh of a cost

Thanks

Nick