Expense Loophole

If you are referring to the expense ratio, Vanguard is a mutual fund company that has a reputation for cheap, quality mutual funds. Dodge and Cox has pretty low expense ratios as well. I am sure there are plenty others, but these two are the first that come to mind. Probably the cheapest type of mutual fund you can get in general is an index fund from any of the major companies.

Buying directly from Vanguard will keep the expenses and fees low.