I can't really see any correlation of difficulty and price. It's related to spiraling hardware specs which cares not for what traders are doing. The traders who dictate the price probably don't care as long it all still creaks along.
Well if you think about it for a while it is quite intuitive. There are 3 reasons why you would not observe a clear correlation even though the basic idea is that for any given asset price will converge to intrinsic value over time.
1. Some assets like bonds fore example are very easy to value because they have more or less predictable cash flows in the future as well as companies like S&P rating the solvency of the issuer. For bonds price and value will tend to deviate little. Bitcoin on the other hand is a different asset more akin to gold with no cash flows one can use for valuation. For these type of assets value a price can deviate significantly over a prolonged period of time. Also value becomes much more subjective and difficult to define.
2. In any case any asset must have value otherwise nobody would waste their time with it so you must think what are the value drivers of an asset like Bitcoin? As a bitcoin trader I have considered this question for long and came up with several ideas one which is the degree of security of the network or how resistant it is to attacks. Of course this is not the only value driver but it is a major one in my view. All other things being equal a cryptocurrency with more mining power backing it will be more valuable than one with less mining power simply because it is more secure.
3. The reason why you'd not observe such a correlation in the short term is because if you would you'd be making money very easily. Buy bitcoins, then buy mining equipment and start mining, price increases, sell come of the bitcoins, buy more mining equipment, make more money, repeat.