The practical part of the SLC assessment will take about 6 hours on the water with a NauticEd SLC qualified assessor.
You can also get access to this assessment document when you login under your curriculum page then under OTW (On-the-water) assessments. You should study this document in detail a few weeks prior to your assessment. If you believe you are weak in any area TELL YOUR ASSESSOR before so that they can arrange a quick training before the assessment. If you show up and try to wing it, you may possibly fail. As well as practical skills, you should also be fully versed on the theory knowledge elements of rules, navigation, tides, weather systems, lights on common vessels, and aids to navigation buoys.
The cost will be for your instructor's time and apportioned boat cost. Typically it is around $100 per hour for the instructor's time and boat but this will vary greatly due to location. i.e. dock rental in California and New York etc are very high. Also if you arrange a friend to join then the cost can be reduced.
The details on how to gain an SLC is here:
The cost of the theory part to the SLC is detailed in the above link. Just quickly, is $175 for the Bareboat Charter Master Suite of courses and a $95 issuance fee.