The total expense ratio (TER) is the measure used to determine how much it costs to run an ETF. Your ETF return is whatever the market delivers minus the cost of running the ETF. We only know the exact TER at the end of a period (usually a year), because even if the issuer can predict its costs exactly, it has no way of knowing how much will be invested in the fund at the outset. This post discusses this in more detail: ETF: Understanding TER
Other related posts:
ETF: Are cheaper ETFs better?
ETF: Fees you can control
Podcast: Figuring out investment fees
Wealthy Maths: Calculating investment fees