at a cost of roughly a tank of gas in a premium sedan.
Roughly, not exactly. Pegging the price of a battery switch to the price of gas really wouldn't make any sense, although it might make sense to make it based on the cost of electricity in the area, assuming that that varies.
If I were to price it I would use the following formula once there were enough stations to make simply swapping a battery without needing to return to the same station to get the original back a viable option:
Price = Cost of electricity for a full charge from current battery state + prorated (operations and maintenance cost to swap + cost to amortize investment in station + cost of station depreciation) + allowance for replacement of bad batteries received in swap + desired profit per swap