You're not anchored to anything. Price dictates mining costs, there's no "unless they can mine at a lower cost", if the market isn't offering them above cost they switch to mining something else. MN operating costs are close to 10% of rewards at the mo (ignoring depreciation on collateral) so not entirely free but it wouldn't really matter. If Dash was entirely PoW the price would be just the same, what's being soaked up by the markets determines the price (in a perfect world ofc, here in the real world it's mostly determined by manipulative trading and selective reporting).