You probably wont' need to replace the battery (assuming you actually do keep the car for 100,000 miles). The battery will merely degrade, the extent of which is unknown, I wouldn't count on GM providing cheap replacement batteries, even if the price per kWh of lithium batteries continues to decline.
I prefer to run the math in terms of ROI...how many miles you need to drive for the car to essentially pay for itself from the savings of not having to buy gasoline, oil changes, and other ICE related maintenance such as timing belt, water pump, fuel pump, etc.. I had a 2004 Audi wagon that I sold last summer that I drove for 105,000 miles. Based on what I paid for my EV...I figure that If I do drive it for 100,000 miles; the car roughly pays for itself...that is of course comparing it that Audi that only returns about 19 mpg and runs on premium gasoline. I'd have to drive a lot more miles to get my money back if I did the same comparison with my VW diesel that returns 40 mpg.
Remember to take the cost of electricity and public charging into consideration. A buddy of mine in Carlsbad drives a Model X with a 90kWh battery. He can pay up to ~$0.40 per kWh charging at home depending on TOU rates, so theoretically up to 36 bucks for a "fill up". Not a ton of savings there vs gasoline. Electricity is comparably cheap where I live. Last month's spot rate was $0.028 per kWh. That same 90 kWh charge would only cost $2.52 here in Alberta.
