Credentials come in several shapes and sizes as well as variations in technology.  Credentials are also a commodity and there are significant price breaks as a greater volume of credentials are purchased.  If you are in the process of designing a system, you should take into consideration the cost of the reader when determining your budget.  If the volume of credentials is low, say 200 or less, proximity cards may be more economical than barcodes because of the price difference with the reader