>>328634835
If that should happen, selling a 3-6 month old Index 1 will almost get all your costs in. I expect any Index 2 Kit to be way more expensive.
All that is assuming that Valve develops a new headset, controller and base station. I'd think they only develop a new headset, which would make upgrading really easy and not as expensive as the market for "spare index items" is rather large.