The price of bitcoin is formed exclusively by supply and demand for it. Bitcoins are not secured by any tangible assets, but the same situation is with any Fiat currencies, rubles or dollars. It is wrong to assume that bitcoin is provided with electricity spent on its mining or the cost of a bitcoin farm. The difficulty of solving the cryptographic problem during mining varies according to the network hashrate used to find the latest 2016 bitcoin blocks. If the number of miners and the electricity consumed fall several times, then the complexity of mining will simply decrease and the process will stabilize again. So the value of bitcoin and its purchasing power lies only in the public's trust in it.
Not the same. The price of bitcoin is indeed determined by what people are willing to pay for it when they are trading (not the best kind of demand and supply, but still). Fiat price is determined by the regulatory authority that sets the price and makes sure that it's under control regardless to what's going on. As for bitcoin purchasing power - unfortunately, I don't think it's appropriate to talk about it for now. There aren't many places where one can pay with btc. And are there are places that determine the prices not by converting the fiat price into bitcoin one's? It's too dependent and too unstable for now.