From Tesla S-1: "We currently expect that the Tesla Roadster battery pack will retain approximately 60-65% of its ability to hold its initial charge after approximately 100,000 miles or seven years"
That's $12k now in order to get a replacement battery in 8 years time, it'd presumably be more if you paid at the time, and it's not clear they're even offering the scheme - that post just says they will "in the near future".
And Tesla itself would disagree as it has stated 70% after 7 years/100k miles.
I'd be careful to compare with the Roadster either way.