It depends on how long you hold. If you bought ARK and held for 20 years, the ER would compound to a significant dent in your earnings.

If you made an initial investment of $1,000 and added another $1,000 each year with an expected annual return of 8%, you'd pay $4,842.56 in fees at an ER of 0.75 with a net return of $49,241,32.

You've effectively given ARK almost 9% of your total portfolio value in fees, whereas a fund with a 0.10 ER would only cost you $674.88 over a 20 year period with the same investment strategy and time horizon.

Now if you expected ARK to average 20% returns over 20 years (which would be insane by the way), the fee difference becomes less of a pain. However, you're still looking at a difference of $25,232.97 in fees vs. $3,515.29.