The NSCA certs cost about $300 each, and you also have to pay $100 a year to be a member of the NSCA. The ISSA certs are about $400-500 each but no annual membership fee. The NSCA is more strict about continuing education units as well for maintaining your certs, they expire after 2 years and you need to go to seminars and classes to get your CEUs. Those cost money as well.
Anyways, certifications are nice and you learn a lot but they are not everything. Expanding your knowledge on health and fitness as much as possible and learning how to market yourself are more important and necessary than certs. They are not required everywhere. Being a Performance Nutrition Specialist doesn't really mean that much, it just means you are competent enough to make nutritional "recommendations" to clients. It does not make you a Registered Dietitian by any means.