Dirt cheap, but good enough
My own belief is that for VR to become mainstream (i.e. wanted and used by a much wider audience than just the tech-savvy and early adopters) it has to be dirt cheap to put test the water, but good enough to make users want more. And there has to be lots of content people want to use.
Dirt cheap because it has to be a low risk/no risk thing to try out for those with a vague idea that it might be fun. Lots of content because average people just want to enjoy it, not get excited by the technology.
Google cardboard is capable of delivering the 'wow' factor at a cost which makes it a throwaway item, provided you have a suitable 'phone. And that means a huge chunk of the population in most of the developed world.
Content can be delivered via YouTube, a very familiar channel. Once users see what Cardboard can do, they'll want more sophisticated headsets and be prepared to pay for them, so it would surely be easy enough for other manufacturers to produce compatible devices and jump on that train.
The main obstacle right now seems to be making it easy (and cheap) to create the content. Cost of a rig and hardware capable of managing the post processing in a reasonable time are obstacles which will stop all but the most determined from becoming content producers for now.