<prompt> Human: My plan is to just put any extra money that I have that I didnt spend or put into a retirement account into a 3 fund index fund 60 domestic 25 international 15 bond. Ill put in around 200 if I spend a lot during those 2 weeks and 500 a week if I spend a little.Are ETFs or mutual finds better for this? Since the amount that I would invest would be variable there probably isnt a way I can automate this, but would there be a way to automate it if I wanted to invest a flat amount and dollar cost average into it?  <response>: What is the best way for me to do this to minimize the cost of buying index funds all the time.There is no cost to buy or sell Vanguard mutual funds or ETFs from within a Vanguard account so this isnt really something you need to worry about. Are ETFs or mutual finds better for this?If you want to automate this then you have to use mutual funds Otherwise either would work. 