By your analysis, bitcoins should have been super expensive when they were being mined by CPUs.

Mining difficulty/expense has absolutely nothing to do with the price of BTC, only the profitability of mining, since the network autocorrects to produce a known amount of coins no matter what mining power is.

BTC price is determined by demand for BTC. Nothing more, nothing less. A mining equilibrium can settle it at any bitcoin price, from .01 USD to $1000 USD (and likely will).