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automatic investments for cheap
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for your purposes, i would recommend using direct investment in a no-load mutual fund.  i mostly use vanguard and would recommend them.  they just about invented index funds, usually have the lowest (internal) expenses for index and many other funds, if you take electronic instead of paper statements there is no maintenance fee, have no transaction commission, can do periodic automatic investment from a bank account etc.  a typical index fund there would require an initial $3000 investment and would have a minimum of $100 for each additional investment.  if you can't come up with an initial sum of that size, you might be able to find a broker with a lower minimum and suitable free etfs trades as others have suggested.
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