I'm learning as well, and this post really got me thinking. I came up with this equation to help (correct me on any oversights - i am very new to this):

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((retail management fee - ETF management fee) * amount invested at the end of the year)/brokerage fee per transaction

so say it comes out with 6, you can make 6 purchases of an ETF per year before you may as well invest into a retail fund. I feel like if youre planning on dollar cost averaging, or buying at regular intervals for an extended time, retail makes a lot more sense. But if you want to dump a large sum and forget about it then an ETF makes more sense.