Nobody really knows all the answers for sure. Especially "how long they last", which is not really answerable. It depends on many factors; and some of them are not in Tesla's control. And they they slowly lose capacity, so it's more a question of when you want more range than when it "dies".
One forum member recently asked what it would cost to replace his Model S battery now; I believe he was quoted over $40k for an 85kWh battery. (That's retail price; Tesla's cost is likely significantly lower. Tesla may be pack-limited so they may be trying to discourage people from doing this now). However, there's no need to replace it now - Tesla has said that unless you intentionally harm your battery, they will replace it free if something goes wrong. (Although it looks like that doesn't cover unusual degradation, but hard to tell for sure). It has an 8-year warranty; unlimited miles if you get the 85kWh. 8 years from now, the price is likely to be significantly lower.
Tesla has said they will pre-sell replacement batteries; under that plan you would pay $12k for a new 85kWh battery in, I assume, 8 years. If it's like the Roadster replacement battery, you pay more if you get a new one earlier, less if you get it later. They didn't say WHEN they would offer that program and it doesn't appear to be available yet.
The "design life" of the batteries is supposedly about 16 years (though Tesla has said very little on this subject). 16 years from now you should have 60% of the battery left; hopefully their numbers are conservative as they were for the Roadster. That's 159 miles if you range-charge an 85kWh. So you may not have to replace the battery at all. Although some time in the next 16 years you might find it worth buying an upgrade. Or just get a new car, and let somebody else have the older car that doesn't mind the lower range...
Cars with older batteries are worth less as long as batteries are expensive (though their value would jump way up after a replacement); that's why Roadster values are so low...everybody is afraid the batteries will die soon (though the Plug In America battery survey indicates that is not likely the case) and that it will still cost almost $40k to replace them. Presumably Tesla will lower the price of the batteries at some point, but nobody knows when or how much. The sooner they announce it, the higher the price has to be because of their costs, so waiting has advantages. I think Tesla should provide another battery replacement program for the Roadster; that way they can price the program low, but not have to actually make the replacement batteries for a few years. That would make people feel more comfortable with questions like these.