May 6, 2013 - 2:06 PM
Your calculations are close, but the Swiss Card is cheaper than CHF 225. Also your return trip does not end in St. Gallen but at the border in St. Margrethen (or perhaps Buchs, depending on the route) which makes it a bit more expensive if you buy regular or half fare tickets.
I would recommend the Swiss Card. It saves you money and it's more convenient as there are 2 tickets less to pucrhase. Also, if you buy it in advance you save time in Basel as you don't need a ticket and can board your next train straight away. Please check the link in my previous post for Swiss Card prices, resellers and further details.
Enjoy your stay!