If a stock trades at $11 and pays a $1 dividend, it will typically end the day trading at around $10. Now suppose all the investors have chosen to automatically re-invest those dividends into the same company. Do they pay the $11 price or the $10 price?
Keep in mind the ex-dividend date is different from the payable date (the day the dividend is paid). That means the market price will already have adjusted lower due to the dividend. Short answer: you get the lower price when reinvesting.
So here's Vanguard's policy, it should be similar to most brokers:
When reinvesting dividends, Vanguard Brokerage Services combines the cash distributions from the accounts of all clients who have requested reinvestment in the same security, and then uses that combined total to purchase additional shares of the security in the open market. Vanguard Brokerage will attempt to purchase the reinvestment shares by entering a market order at the market opening on the payable date. The new shares are divided proportionately among the clients' accounts, in whole and fractional shares rounded to three decimal places. If the total purchase can't be completed in one trade, clients will receive shares purchased at the weighted average price paid by Vanguard Brokerage Services.
If a stock is trading for $11 per share just before a $1 per share dividend is declared, then the share price drops to $10 per share immediately following the declaration. If you owned 100 shares (valued at $1100) before the dividend was declared, then you still own 100 shares (now valued at $1000). Generally, if the dividend is paid today, only the owners of shares as of yesterday evening (or the day before maybe) get paid the dividend. If you bought those 100 shares only this morning, the dividend gets paid to the seller (who owned the stock until yesterday evening), not to you. You just "bought a dividend:" paying $1100 for 100 shares that are worth only $1000 at the end of the day, whereas if you had just been a little less eager to purchase right now, you could have bought those 100 shares for only $1000.
But, looking at the bright side, if you bought the shares earlier than yesterday, you get paid the dividend. So, assuming that you bought the shares in timely fashion, your holdings just lost value and are worth only $1000. What you do have is the promise that in a couple of days time, you will be paid $100 as the dividend, thus restoring the asset value back to what it was earlier. Now, if you had asked your broker to re-invest the dividend back into the same stock, then, assuming that the stock price did not change in the interim due to normal market fluctuations, you would get another 10 shares for that $100 dividend making the value of your investment $1100 again (110 shares at $10 each), exactly what it was before the dividend was paid. If you didn't choose to reinvest the dividend, you would still have the 100 shares (worth $1000) plus $100 cash. So, regardless of what other investors choose to do, your asset value does not change as a result of the dividend. What does change is your net worth because that dividend amount is taxable (regardless of whether you chose to reinvest or not) and so your (tax) liability just increased.