Historically, difficulty and exchange rate have shown correlation. However, correlation != causation.

One could argue that increase in difficulty helps drive increase in value, but difficulty is only one of the many factors that must be considered.

For the past, present and near future, the most important factor, besides difficulty, that affects BTC exchange rate is speculation of its future value. These two factors, are the major drivers of exchange rate. Alas, while difficulty is a concrete measurement, speculation of future value is not so easily measured.