How big a job is it to put in another sink in a kitchen that only has one sink and how much would a job like that cost?

How big a job is it to put in another sink in a kitchen that only has one sink and how much would a job like that cost?

In the same area or somewhere else, like in an island or across the kitchen?

Better explanation needed as there are lots of questions.

So do you want two separate sinks or are you thinking a double sink? What are your counter tops made from?