To be fair, the value of securities like PETA is mainly speculative, since the underlying value is unknown and is in constant flux.  When the difficulty goes up X%, the asset value [hardware] goes down ~X%.*  We don't have enough info to predict difficulty.  Throw in irrationality and illiquidity of the market, and market price becomes the only interesting indicator.

*Actually a bit more, since there's a floor below which the hardware stops generating profit, i.e. (the cost of hosting+electricity) becomes greater than (the coin mined).