It is called variable pricing - and the service you provide shouldn't change based on what keywords are relevant. It is not like BMW vs Kia whatsoever.
I don't claim to know all about SEO, but the basics are incredibly simple. I do know there is an abundance of people offering services like this, and they like to spread a lot of FUDD.
- charging different for different keywords is called capitalism; some keywords are BMW's and others are Kia's now go tell bmw they can't charge more
I don't pretend to know all coding, don't pretend to know all SEO; don't fear other's skill sets.