Rationale:
1. You will (over time) benefit from the daily fluctuations in price
2. Assuming the buys are made on an exchange the only fees will be the exchange fees for each buy - these are likely to be on a percentage basis and therefore indifferent to one $1400 buy or seven $200 ones;
3. withdrawals to your wallet can be done weekly or monthly as you prefer to minimise network transaction fees
If you do Dollar Cost Averaging, you won't care too much about fluctuation. It is a strategy for people who don't have time to follow market or don't want to have pressure from daily changes of price.
If you let price fluctuation affects your decision and DCA action, you are trading, not DCA.
People advised very good above that min amount to do DCA should be significantly bigger than trading fee and withdrawal fee. Because you should move your Bitcoin by withdrawal from exchange to your own wallet.
Not your keys, not your coins.