You can value an equity based on it's discounted future cash flow discounting by a risk-free rate (typically US 10 yr notes). QE is the Fed making large purchases of t notes on the open market to keep the rate low, in order to keep capital cheap (and ostensibly encourage investment). If they stop these purchases there is a widely held belief that the t note rates will increase, which will hurt the equity premium across all companies. You can play around with any number of free online DCF calculators to see how varying the risk free rate of return effects current valuations.
edit: I am addressing the general question, not ipo valuations.
edit: I am addressing the general question, not ipo valuations.