I am buying Vanguard ETFs on e-trade. For the purposes of this question, suppose I am buying VO (a multi-sector mid-cap fund). It is a popular symbol (share class total net assets: 51B).
I typically cannot get to my stock account during market hours. My account does not have the Extended Hours feature. So, when I place a market order before market open, e-trade holds the order and executes it when the market opens. Since this is the most volatile time for the market, I expect that sometimes I might not get the best price. If I do this repeatedly (say, an equal investment once a month over many years) how much more will pre-market orders "cost" on average (due to volatility) compared with waiting until 30 minutes after market open every time I buy?
"cost" here could be volatility, unfavorable price, or anything else that people are concerned about when they recommend against extended hours trading. Please quantify this cost for the specific example of buying VO before market open on e-trade.