The NAV, net asset value of all the companies the ETF holds, is calculated at the end of every day. Therefore, an ETF can be sold at a discount, lower than current NAV, or premium, higher than current NAV, of the underlying securities. Unfortunately, the NAV is not calculated throughout the day. Therefore, the discount/premium compared to the underlying securities is inaccurate.
Fortunately, there is an AP (Authorized participant). His/her job is to make sure that the ETF price accurately reflects the NAV of the underlying securities throughout the day. Therefore, I will not be concerned about overpaying for an ETF. Nevertheless, I always use the limit order when buying or selling ETFs.