However much you hate it, the bottom-line finance number gives you an idea of the materials, work, availability, etc. involved.
A system that is not economically viable is taking MORE product out of the earth, and rarer products, that need more refinement and processing, etc. in order to create it in the first place than it is replacing other power-generation methods and their costs.
It's quite simple. The market price changes to reflect the difficult, cost, legislation, rarity, etc. of the materials and labour involved. If something is more expensive it's because it COSTS MORE to give it to you. If something can't pay that cost back (at least, in a reasonable time) you've taken out MORE from the earth including shipping the thing to yourself and paying for machines to modify it, and paying for the companies mass-production plans, etc. than you've stopped being taken out elsewhere.
It's not perfect. It's not entirely accurate. But the monetary cost of something is a pretty good indicator. This is why lithium batteries are more expensive than lead-acid equivalents, why oil products are being taxed, why discovery of shale gas can drop the gas price, etc.
Also, as you're moving the burden from government and entire countries to individual users here, cost matters more than most other things. You're asking ME to take the effort, research, purchase, maybe pay for planning and electrical works, etc. this product that you're SELLING in order for me to help the earth. There's a cost involved in that no matter what. Some of that cost is a "donation" because you want to live in a friendly way. Some of that cost is because of the convenience to you if the power blips for a moment. Some of that cost is for your peace of mind.
At the end of the day, cost is a pretty good measure of all kinds of things to do with a system. This is why energy companies are complaining about the "payback" electricity schemes from solar users... the costs they incur to put their pittance of electricity back into the grid far outweigh anything else. The government has to subsidise those costs, or the electricity companies have to raise their prices. And, suddenly, it's actually more expensive to run "off-grid" than you thought and you end up going back "on-grid" because the cost isn't worth the convenience any more.
I could UPS all my appliances today. I could just buy a tiny UPS, or save up towards a bigger one, each month and stick them on batteries that survive power outages for whatever length of time I choose to do it for. But I don't because it costs. And that cost does not compare to the cost of the power going off every now and then, or the electricity company raising its prices by 10% a year.
If an off-grid system does not return money for you, the money you pay would have been better off just buying a generator and some fuel for it for the rare occasions the power does go off, and forgetting about all these fancy gadgets that help you live off-grid. In which case, both the green-ness and the user suffer.
That's why governments are subsidising PV etc. installs. They have to bring the price down or people will just look and think "Sod it, I'll just buy a genny and keep a tank of petrol in the garage for if anything happens" rather than go off-grid.
Things have to be profitable, and everything has a cost.