Many fund companies make it easy for small investors to invest in Mutual Funds using automatic investment plans, especially for retirement accounts. Generally low balance fees and account maintenance fees are also waived. Automatic Investment plans also have the added benefit of creating a sustainable investment plan and building savings without too much effort.
A month or so ago, I sat and compared plans from several major fund companies and I found that T. Rowe Price has the best automatic investment plan amongst them all. The plan requires a minimum contribution of only 50$/month per fund and has no custodial or low balance fees of any kind for IRA accounts if you select any of their managed funds (non-index funds) and keep contributing until their regular fund minimums are reached.