Roths are very flexible depending on which company you choose to put your money into. You may be able to start with as little as $100 with some companies! Here are some of my favorite low-cost investment options. I've broken it down by the dollar amount you need to get started.

$100 and Up 

Schwab Total Stock Market Index Fund
schwab.com
Invests in 4,000 companies
Expense: 0.09%
Minimum investment: $100

Schwab International Index Fund
schwab.com
Invests in 350 publicly traded companies from selected countries outside the U.S.
Expense: 0.19%
Minimum investment: $100

Schwab Small-Cap Index Fund
schwab.com
Invests in the second-largest 1,000 publicly traded U.S. companies
Expense: 0.19%
Minimum investment: $100

$1,000 and Up

Vanguard Star
vanguard.com
Invests 35% in bonds and 65% in stocks using a basket of Vanguard funds
Expense: 0.37%
Minimum investment: $1,000 minimum to open an IRA; $1,000 minimum to open an investment account

Vanguard Target Retirement Funds
vanguard.com
Invests in a combination of stocks, bonds and cash equivalents
Years available: 2015, 2020, 2025, 2030, 2035, 2040, 2045, 2050, 2055, plus the Target Retirement Income Fund for those already in retirement
Expense: 0.21% or 0.22%
Minimum investment: $1,000 minimum to open an IRA or general account

TIAA Equity Index
tiaacref.org
Invests in approximately 3,000 companies
Expense: 0.09%
Mininum investment: $2,000 to open IRA; $ 2,500 for general account (or $50 if you start an Automatic Inv. Plan). If your balance is below $1,500, you will be charged $25, unless you invest at least $100/month in an automatic investment plan.

T. Rowe Price Automatic Asset Builder
troweprice.com
Expense: This is simply an automatic investement program, so expenses vary depending upon which fund(s) you actually select.
Minimum investment: $1,000 minimum to open an IRA or general account

T. Rowe Price Target Retirement Funds
troweprice.com
Invests in a combination of stocks, bonds and cash equivalents
Years available: 2010, 2015, 2020, 2025, 2030, 2035, 2040, 2045, 2050, 2055
Expense: between 0.67% and 0.84%
Minimum investment: $1,000 minimum to open an IRA; $2,500 minimum to open an investment account (Editor's note: This option also available via the Automatic Asset Builder program as noted above)

$2,500 and Up

Fidelity Four-in-One Index Fund
fidelity.com
Invests 48% in 500 largest stocks, 12% in rest of U.S. stock market, 25% in international stock index, 15% in bond index
Expense: 0.23%
Minimum investment: $2,500 minimum to open an IRA; $10,000 minimum to open an investment account

Fidelity Freedom Target Retirement Funds
fidelity.com
Invests in a combination of stocks, bonds and cash equivalents
Years available: 2015, 2020, 2025, 2030, 2035, 2040, 2045, 2050, 2055, plus the Fidelity Freedom Income Fund for those already in retirement
Expense: between 0.71% and 0.91%
Minimum investment: $2,500 minimum to open an IRA; $2,500 minimum to open an investment account

Vanguard Total Stock Market Index
vanguard.com
Invests in approximately 3,400 companies
Expense: 0.18% (other fees may apply)
Minimum investment: $3,000 minimum to open an IRA; $3,000 minimum to open an investment account