It depends on the mutual fund and the ETF. For Vanguard 500, the VOO ETF and the VFIAX index fund are effectively the same, they even have the same expense ratio. The latter you need $10k upfront though. (VFINX, if you only have between $3k and $10k, has a higher expense ratio.)
VFIAX is not a mutual fund, it's just the "Admiral" class of the VOO ETF. Admirals shares offer lower expense ratios in many instances, but I guess the VOO expense was already at its floor.
VFIAX is a mutual fund. Vanguard holds a patent which allows a dual share class, where the ETFs and mutual funds share the same pool of securities. This allows share conversions from mutual fund->ETF, interestingly.
Not exactly a good argument for mutual funds being the same cost as ETFs though. It's only technically a mutual fund from what I can tell. I see your point though.
Vanguard's Admiral share mutual funds almost never differ in cost from their ETFs.
As for it only "technically" being a mutual fund: consider that VFIAX predates VOO by 10 years. How does that square with VFIAX only "technically" being a mutual fund, when presumably it was a real, honest mutual fund in the years 2000-2010 (before VOO was introduced)
