Each SIM card will cost $10, with a $10 account credit once activated. Best Buy staff will not assist in activating the SIM card; the store is only a retail outlet for you to purchase a SIM instead of having to wait a few days, or in my case, weeks, for a SIM card to arrive before setting up service.
One important detail to note is that the SIM cards sold at Best Buy cannot be used with data only plan. The cards will only work with Google Fi Talk & Text plans, which start at $20 a month for unlimited voice and text, with data costing $10 for 1GB, up to 6GB. After the 6GB mark, Google will slow down the data connection once you reach 15GB.