Adding docling collars also give some redundancy to the ISS. And as for cost , cheap at the price, the trick is getting those russians on schedule. And let's not even go into the fact that all you really need for a rescue vehicle is a heat shield, parachutes and personal air supply for each returnee. [reference: The end of the Movie Dark Star is technically doable today and comparitively cheap.]

Never watched dark star, but youre right, soyuz capsules would be cheaper and work just as well, except for the fact of when you finally get a full 7 person crew aboard the station and you have 3 different capsules, that also means there are 3 different places something can go wrong instead of one. Personally, I think they should just permanently park a shuttle next to the station. It would add to total station space as well as act as a lifeboat.

A use for SpaceShipOne? The first reusable orbiter will get quite a few customers, but in the meantime, isn't there a way to boost a sub-orbiter up to ISS as a lifeboat?