A. Unlike traditional mutual funds that trade only once a day, ETFs trade on the exchanges continuously, which means there are times an ETF's price could be off to its real value, a lot.
For example, on August 24, the S&P 500 fell 5.3% at the opening, but the $65 billion iShares Core S&P 500 ETF (IVV) fell as much as 26% before recovering!
There is a way to find out the intraday net asset value of an ETF, here is the trick:
For any ETF, for example, IVV, type "^IVV-IV" at Yahoo Finance, you will see IVV's intraday NAV, by seeing the NAV, you can make sure you are not overpaying the ETF.
Knowing what's the true value of a fund is just the first step, next you need to execute the buying or selling order well.