An abortion at 20 weeks is not much different than an early abortion, except that the preganncy is more advanced.
They use suction aspiration to suction the pregnancy, and they also use forcepts and a curette to ensure that all tissue is removed so that no comlications occur.
You can be put to sleep for this.
Different clinics have different prices.
gynpages.com
Has lists of clinic and their websites, you can see how much it costs here.
Some states, if you are willing to travel, offer lower cost later abortion services.