The simplest option I think is just to open an account with Vanguard and invest in a 2050 retirement fund. However, the 2050 fund has an ER of .19% and according to VG's website:
Let's say I have 5k to invest, that $20 fee is equal to a .4% fee which I think makes Vanguard the most expensive for this small amount of money. Total ER would be .59%2 A $20 account service fee is charged annually. The fee is waived for Voyager, Voyager Select, and Flagship members.
Open an account at TD-Ameritrade where I can invest in no fee ETF's. They offer Vanguard funds too so I could potentially save money on ER by investing with TD-Am until I hit 10k. This option is not as easy as number 1 or 3 since I can't invest in Vanguard retirement funds so I'd have to re-create it using their no fee ETF's but it would be the cheapest. Does TDA have lifecycle funds to invest in and if so are they free?
I just found out about betterment but it seems very easy and their .35% fee seems pretty low if you are investing small amounts of money. Their total fees would only be .35 plus avg of whatever funds you're invested in.